hmm...ok, here we go:
3.x D&D/d20 Modern:
I wouldn't stay away for fear of combat. It's not really that overly complicated. It can be as strategic as you want or not. If you don't use a battlemap or board and just keep it abstract, then you can move it along fairly quickly. Also, as the GM you don't really have to have every statistic about a creature to run a combat; you can just wing it with a few numbers. All you really need for combat is Hit Points, attack bonus total, and Armor Class. Sure, if you tried to run a 100 person fight with individual initiatives and combats, then it bog down. However, you really don't need to do that; you could just assign group of say 25 each or all 100 and give them the same initiative and combat rolls. You can scale D&D combat up or down as much as you want. At it's core, D&D combat is fairly simple.
As far as money goes, all you really need to play is the three core books: Player's Handbook, Dungeon Master's Guide, and Monster Manual. You can usually find these used and new on eBay and Amazon for reasonable prices (half or less of original price). As it has already pointed out, you don't even really need the core books: the 3.5 D&D SRD is free online, and so is the d20 Modern SRD.
The links to the 3.5 Dungeons & Dragons SRD and the d20 Modern SRD are on this page (left side under System Reference Document):
The SRDs are all the rules for the core books, just boiled down and without fluff and pictures. All you really need to play these games are contained in the SRDs. If you are into tinkering and world building, then these are the perfect platform because you can use these SRDs to tailor your own fantasy, past, modern, or futuristic games and campaigns. The 3.5 D&D SRD is the three core books boiled down. The d20 Modern SRD is the core book, the Urban Arcana campaign book, and the d20 Future book boiled down.
Seriously, if cash is tight, then all you need is the free SRDs to start. If you are the type that needs a physical book (I'm like that too), then I would recommend buying used online. You can usually find good deals and the books will more than likely be in good condition.
As I have said before, you cannot go wrong with GURPS as your first game system for a number of reasons. Unlike D&D, which may go through another edition change in a relatively short time, GURPS 4e remain in its current edition for years to come; if the past is any indicator. The books are hardbound and sturdy. All you really need to play GURPS is the one core book. The great thing about GURPS is the quality of the material. Whereas there have been many D&D books produced, there is a significant balance issue between the books produced after the core books. This is not the case with GURPS. Steve Jackson Games products are thoroughly playtested and researched before they are released.
As far as combat, GURPS can be as crunchy or streamlined as you like because you can tailor it to your liking with options or not. This is similiar to my take on D&D combat above: if you use a battleboard, then GURPS can bog down as well with large combats. But, you don't need to get that detailed; you just can keep it abstract and keep tracking of effective combat skill, DP/DR, Move and Hit Points. If you are focused more on role-playing and less combat, then all you really need are a few numbers for either D&D or GURPS combat. However, you can always make both systems combat more detailed if you wish.
As far as cash goes, GURPS is the better bargin for your money for several reasons. I already talked about the physical quality of GURPS books. Also, the material quality is top-notch and is regarded as the highest in the RPG industry. Any GURPS sourcebook you purchase will have a longevity in your RPG collection because it can be used as a resource for any RPG system that you have or will own. Also, the differences between 3rd and 4th editions GURPS are not all that great, which means that will very little work you can use any of the 3rd edition GURPS sourcebooks with your 4th edition GURPS game. This fact shouldn't be overlooked: 3rd GURPS has a large product line which is composed of quality material. You get the best of both worlds here: quantity of quality sourcebooks that can easily be converted to 4th edition GURPS. Again, you can easily find 3rd GURPS now used online for relatively cheap because now there is a new edition. If you are looking to maximize your spending power, then this is an avenue you should consider.
Also, with GURPS, all you really need to play the game is the Lite version, which is also free to download. It is just a streamlined version without all the myrid of choices; which could be a bonus starting out with the system. One of the most time consuming thing about GURPS is character creation due to the choices. However, this is GURPS strong point: characters are very detailed and can be tailored made to focus on role-playing and not just a bunch of combat statistics like D&D. If you are just using the Lite version, then you can reduce the number of choices at character creation which will reduce the time in creating characters for both the players and the GM's NPCs.
GURPS Lite can be downloaded here:
Both systems have merits and flaws. Both systems are good for what they were created to do as a game. It comes down to a fundamental different approach: D&D/d20 Modern are class-level based systems, and GURPS is skill-driven point assignment system. D&D/d20 Modern character creation is quick and dirty while GURPS is more detailed and character driven. Combat for either game system can be as abstract or detailed as you wish.
Considering quality, logveity, and money spent, I would say that buck for buck GURPS is the better choice. GURPS has been around since the beginning of the RPG industry, and has set the standards for quality in the industry. GURPS is one of the top 5 game systems of the industry. More RPG produced are really just a variation of a select few systems: they are either skill-driven percentile based (Basic Role-Playing), class-level based (Dungeons & Dragons), or point-based (GURPS) when boiled down.
Don't get me wrong, I love both systems for different reasons. I have been playing D&D in every edition since 1982 (well, technically: 1st and 2nd AD&D, 3.0 and 3.5 D&D). I played 3rd GURPS for almost 10 years. My group of friends gravitated back to D&D when 3.0 Dungeons & Dragons came out and we haven't gone back to GURPS. However, I would play GURPS again in heartbeat if the group would go for it.
GURPS- GO FOR IT!!!!