For your first piece of "old iron" saw, I would go with either a Super XL or XL12. Simply cause of the abundance of parts on the market, and a decent unit can be had for less than 100$ cause of their abundance. They are one of the few you can get a new Piston/Cylinder kit for. They are also repoping some of the fuel line grommets and older XL12 step down fuel lines. New reed cages are available for Super XL, not sure about XL12. Homelite also flooded the market with replacement parts during their long production run, so there is plenty of NOS stuff out there too. Only down fall is bar mount. It is not very common with it's 3/8" studs, but you can make other bars work with a file.
Second would be something like a 10-10. Once again, abundance. If you manage to get a bad one, you can get another one and have a parts saw, and once again, I see them on CL around here for under 100$.
There is a guy advertising a good running Mac 700 for 65$ on CL in Mooresville NC, if it is not sold yet...