Surprised no one has suggested getting an Aero lower and a Palmetto kit.
At today's prices, the lower would run 71.39 at Brownell's (and you could find cheaper lowers out there, I just like Aero Precision) and an M4gery kit (blem) from Palmetto is 360. Total would be ~430, plus another 70 for S&H and the FFL should put it right at $500 for a reliable and decent quality AR. Get a part time job on the side and get you some toys.
My primary firearms love remains duty/surplus handguns, such as the CZ 75, Sig P226, Beretta 92, etc. All nicely made, all like to eat the cheapest/most common pistol caliber (9x19), etc.
I have surplus rifles, mostly bolt, and SKS. I have purchased several "based on" firearms, which aren't REALLY surplus, but are 'close enough' for me. A couple USGI/milspec-type 1911's, instead of (before) the CMP-released actual surplus pistols. A Century C308, which has a lot of Cetme parts and follows the design (and combines that with the very similar HK G3). If I'd have got in the game early enough, I'd have picked up one of the FAL options. I chose the SKS over the AK, in retrospect I would have done both. I say that now, but funds were still tight... but I would have made it work.
I've put together some budget ARs, wasn't really satisfied until I could make one look "right"- the best I have managed is an M16A4 clone. I'd really like to put an A1 or at least A2 together.
Once you get outside the surplus/clone part, and start looking at the utility aspects of the platforms, it seems like the AR and 1911 platforms are the base to start with. 1911's are built into race guns, target guns, all sorts of different calibers.
Same thing with ARs. You can get your 10" barrel and brace, and make a nice, handy little 'truck gun'. You can put a fixed stock and 20+" barrel and maybe a different caliber, and have a very accurate mid-distance shooter for hunting etc. You can put all sorts of optics on it, from a $20 red dot to a $1500 real ACOG, or true high end scope; you can go with fixed or flip-up iron sights. The platform is incredibly flexible, and if you're building on a budget to "just have something before they get taken off the market" you have a ton of options. You probably can spend over 3k to get the coolest name brands, or you can buy the cheapest upper and lower combos to put together a functional rifle under $400.
Just my own opinion, but if you're even halfway on the fence about getting one "just to have", you might as well grab a complete lower for under $150 (probably PSA), a complete 16" upper from BCA for $250, and a case of Tula steel-cased 223 for $350.
If you want an AR pistol for just-in-case, again, the PSA deals will often go under $150 for a complete lower, and the BCA uppers will be as low as $200, depending on style (they run the side-chargers lower than traditional rear-charging with shell deflectors).
If you want to try x39, BCA has them at the same price or cheaper than the 556 models, and Targetsportsusa can sell you 500 rds for $175+ free shipping. Reliable magazines for this will cost more than standard mags, and 7.62x39 will likely rise higher than 223 down the road, so keep that in mind. But if it's going in the guncase just to have one, you'll do fine either way.
My brother bought an AR prior to Obama taking office his first term, for this same reason. Got an M&P Sport II for $500 and a case of Wolf Gold for $300, over a decade ago. Still has most of that ammo, he shoots a box 2-3 times a year. He's happy he got one (especially now), but hasn't worn it out. Ammo costs more now, the guns can cost less.