$500 is a pretty modest budget. I'd bet that with my Dillon 650 with casefeeder set up for multiple calibers, I have $1500ish in just Dillon equipment, not including dies.
The 550 doesn't seem cheaper until you price the conversion kits (you need one for EVERY caliber to get the casefeeder parts), more expensive tool heads, casefeeder rings, etc. The 550 is much cheaper to set up overall.
I would not bother adding a case feeder to a 550. Without auto indexing it's a useless feature.
If you're only doing pistol ammo, don't forget about the square deal press. It uses its own dies and only does pistol cartridges but it is cheap and auto indexing.
Also, the 1050 may seem like it can be set up for a thousand dollars, but remember that it only comes with one priming system, it is even more expensive than the 650 to convert to other calibers, and it doesn't have the same warranty as the 550 and 650. I would LOVE to have a 1050, but I don't, because I shoot just as much 9mm as 45 (perhaps more), and the 1050 really is for someone who shoots one caliber a lot, rather than a solid mix of many calibers.
I sold my 550 and got a 650 in 2012. I don't think it was a terrible decision as I intend to shoot for the next 50 years, but I'm not sure that I'm persuaded that it's really worth the additional hassle. It IS faster, and when it runs, it produces a lot of ammo really quickly, but it is much less forgiving of problems (it's harder to clear problem brass from the shell plate, the priming system puts out a primer whether the case is there to be primed or not, auto indexing can work against you sometimes if you're not careful, etc.). Maybe I'm spoiled and forgot how slow my 550 was comparatively. And don't get me wrong, I'm happy with the 650. I'm just not sure if it really was worth the price premium, and if I hadn't run a 550 for years, I'm not sure I wouldn't have made a lot of really crappy ammo with the 650. Getting used to the 650 is essential, watching everything carefully is part of operating it every single time, and if I hadn't already used the 550, I'm not sure I'd be ready to deal with its occasional quirks. Then again, everyone said that about running a 550 before I got mine, and said I should start with a single stage, and after I got the 550 and got it set up, I thought most of those people were full of shit, so take what I say with a grain of salt.
If strict budget is a concern, and you want to load a few calibers, the 550 is probably a better choice. If you shoot 2k rounds of one caliber a month, however, the 650 is a much better option. For me, I tend to load 2-5000 rounds of pistol ammo in a sitting and then switch calibers and load something else, so that I can hold myself over for 6 months or so before loading that caliber again. The main benefit of the 650 for someone like me is simply time--the 550 did the same thing, it just took a little longer.