As a long time user, I would say that the M800 is an exellent enlarger. Using an 80mm lens for 6x6 format with the Unicon 105 condenser will make your image a bit less bright, but should not cause noticably uneven illumination. As you noted, the Unicon 85 would be a better match.
The main problem presented by starting off with an M800 now is the lack of easily available accessories, of which there are a wide variety. Ebay used to be a source, but seems to have dried up the last few years. Note that accessories for the M700 will also fit the M800. Lensboards are easy to find; Unicon condenser sets less so, and Setibox diffusion mixing boxes [my preferred] are getting hard to find.
VC filters should not be that expensive if you shop used. You can use the 4.75 inch square size in the filter tray or under-the-lens. The preferred dichro color head is the Cls 80, although the Cls 70 for the M700 and at least one earlier unit will fit as well.
When Durst pulled out of the USA consumer market some years ago, a NY company bought all of the US warehoused parts and accessories. I don't remember the name of the company off hand, but I can probably find it for you if you want it and get back to me. They run a website with an extensive inventory of new Durst parts and accessories still available for older enlargers including the M800, but they are a bit pricy.