Finding a MF camera and couple lenses for under $2k that you can add prisms and maybe a digital back to later means to me a used Hasselblad V kit, or used Mamiya RZ or 645AF. You could probably get an RZproii and 3 lenses and still have money left over. It's really a great value actually, and lots of digital backs were made to fit that camera or can be adapted to fit that camera. You could get into a used 6008AF or integral2 and one lens kit certainly, but pushing the budget. Oddly 6008I2 and 6008AF's are selling new now for close to what the used fetch on ebay, and the used prisms are cheap, but the lenses still command high prices - and way over your budget. As pointed out, there are not as many digital back options for the Rollei 6000 series. For info sake, the best digital set up for the 6008af was the phase p20, and I've seen a number of them sell around $5k for camera kit and back, but you still need lenses. If you can find a CF-22 or CF-528, those are also pretty nice since you don't have the image bank, and not as expensive as the CF-39's.
I like the RZ and don't really mind the weight, but never understood the Contax 645 for its relatively small viewfinder - though I understand that people really like them. The 645af? Never liked it. The mamiya 7ii has great lenses and is a super film camera. It's too bad there wasn't a digital option for this camera. I've seen a few self made ones though. It's fun to see what people craft for themselves. Actually, I'm surprised to not see more adapted phase backs onto the 6008's. There was a poster here years back, Andre Napier, that made himself a p25+ and Rollei 6008AF combo. Maybe as the prices come down on the older used digital backs we'll see more creative stuff being done? I use my AFi-ii on my Linhof Technika and its really fun, but a bit too heavy.