Andrew,
That's a lot of scanning. One option a fair number of people have tried, successfully, is not to scan their own stuff, but send it to a commercial service such as Scancafe. Their basic package is reasonably priced, and from what I've seen of their output, it's pretty good. Between their basic and pro packages you can most likely get the quality you need. Their pricing is arranged so that to a considerable extent you pay for what you wish to keep, not necessarily everything they scanned. 2000 of their scans at a mixture of pro and basic pricing would cost you the value of a good scanner, but the time saving would be immense. Worth thinking about.
If you wish to do all the work yourself, which is the only way I would handle my stuff, because I want total control over the image at every stage of the work, the Epson V750 (this is the model with the fluid mounting accessory and a better version of SilverFast than what comes with the V700) would be the way to go for handling both the 6*6 and 35mm. It's resolution is considerably better for the 6*6 media than for 35mm. For 35mm, it's passable, but you'll get noticeably better quality using the Plustek 7600i. It has very decent resolution for the price (about USD 450). This website has published my review of the Plustek - you may wish to have a look.
If you want to be a bit extravagant and buy two scanners, a combination of the Plustek and the Epson would set you up very well for what you wish to do. You can also consider buying the Plustek and sending the medium format to Scancafe, or buying the Epson and scanning it all at home, or sending batches of the 35mm to Scancafe.
I would also recommend going to the used equipment sites and looking for a Minolta Dimage Scan Elite 5400 model 2. This 35mm film scanner is gem. (Their model 1 is also excellent, but very slow.) The resolution is 5400 PPI - higher than Nikon's flagship 5000 and the lens quality first-rate. While the second-hand Nikon scanners sell at a huge premium over their original retail prices new, the pricing on the Minolta is far more reasonable and the quality excellent. Both are discontinued, but servicing on both will probably remain available until the service outlets run out of spare parts. You'll be able to drive them on the newest operating systems because SilverFast and Vuescan have up-dated drivers, which the original manufacturers have stopped doing. Minolta also had a comparable model - Dimage Scan Multi Pro, a very high quality dedicated film scanner for handling both 35mm and 6x6. You would do very well to pick-up one of these on the after-market as well, but they can be expensive.
Well, there's a range of thoughts for you. No one resounding answer here; just a number of suggestions - it is a difficult area these days because so much good equipment has been discontinued.