So there are a lot of 3DR Solos on eBay. Is it still a valid purchase? I know it is no longer officially supported, but i saw there are conversions you can do to make it work with latest AP versions. I don’t know the history totally, but I guess there were some performance issues?
Anyways, If I can get a Solo for under $200 is that a good investment?
with open solo 3, you cut the ties to 3dr and run solo on 3.5
replace the GPS with a Here, and the original cube with a PH2.1 green cube
and all the issues with solo as sold are non existent
That depends on you. The original Solo was sold as an appliance. No knowledge required, a tool for videographers.
Now it is suddenly very different. No factory support and definitely Do-It-Yourself. It is still a valid machine but you must be willing to ferret out the support information from the various sources on the net.
But technically it’ll fly out of the box right?
Technically,maybe.Depends who had it before you.But some guys buy them up,fix them up (normally an easy fix like software) and bang them out again.So it’s a bit of a lottery.At that sort of price you really can’t go wrong.I imported three from California I was so impressed and it’s not like I’m short of copters.
As Jagger says it depends on what you mean by out-of-the-box. If you mean brand new then it will have the limitations that the original had. It is a camera drone so it is heavily dependent on GPS position. It came with a poor GPS module and hence had some problems. This problem, as all the original problems, can be DIY fixed by replacing the GPS with a newer one. But this requires knowledge, or at least the ability to find the youtube videos on how to do this and to take it apart.
You might read my little essay to get a few more answers to your questions:
https://www.rcgroups.com/forums/showthread.php?2967927-3DR-Solo-raised-from-the-Dead-by-the-Open-Solo-Project
Awesome, and I suppose if I upgraded it I could resale it for a profit
Ooh.That’s another maybe. 
Has to be said that currently trhe batteries are the weak point with a Solo.They are all getting to be old (at least 2 years) and are fading fast it seems.A solution to this will no doubt happen when it gets critical but home brewed battery rigs are appearing.Lots of development going on behind the scenes and a lot more than 3DR ever put in.Check out the Open Solo stuff.
http://ardupilot.org/copter/docs/solo_arducopter_upgrade.html
I have never seen any tech that is a profitable venture from retail