Pretty much everyone will say get a a1200, since its cheap and more modern out of all the amigas.. A4000 would be nice but more expensive and harder to come by.
For me I have been a a500 owner back in the day, but I always wanted a a1200 when I was younger parents didnt have the money.. So a1200 was like the holygrail to me.. Seriously the a1200 is a workhorse compared to the a500..
I say, grab a a1200 if you can for around $105-ish USD. While your at it get a 030 accelerator , budget for around or just under $250USD, then you will have a nice machine where you can push the limits even further than the a500... AGA is very nice to have.. If you pimp out the a500 to the max, it will end up being like a a1200 but without AGA anyways and more expesive especially for acceletors.. I actually I think the a500 is better to pimp out if your into electronic design.. ie make new hardware for it and test it. theres alots of schematics for memory addons, basic ide interfaces, kickstart emulation eeproms, etc.
Meanwhile get a a500 as well, I've seen some on my local ebay from time to time go for $55 USD basic systems.
Also check amibay.com. If your patient and look everyday, Im sure a1200 $105USD and a500 $55 USD are doable.. well I got my a1200 for that price and a500 I got for free some ppl's garage 10 years ago
Not sure how much you are bidding for, but I waited 2 years for a cheap a1200 to come by.. but I wasn't in a rush..
Good luck!