You can have top of the line gear.
You should get a use Contax 645 system. It has useable, accurate AF, is very durable, has top-line Zeiss glass including some of the best lenses ever made in MF, and is very reasonable on Ebay. You could probably score a body and four or five lenses for $6K. This leaves you$19K for a back.
The Phase p45+, which gives 39mp AND 1-2 HOUR exposures, essential for creative landscape shots, can be had on ebay for $13000 to 16000 and maybe 17,000 from a good dealer (maybe less). There is also the p30+ option at 31mp and much less expensive. Only Phase "plus" backs give long exposures so REALLY take note here! I find long exposures essential to my most creative landscape work. All other brands can only pride themselves on 1-2 MINUTE exposures. There is no contest as to this feature, Phase stands alone.
Assuming $17K on a back, you would have $8K left. For that, you get the contax body/finder with 80mm 2.0 (1200-1500), 35mm 3.5 (1200), 45-90mm 4.5 zoom (2500), 120 Makro (1300) and 210 tele (1000). You have a little left over for a flash, filters, vertical grip, viewfinder magnifier to nail focus, etc., making a complete system. You also have the option of single focal length 45mm and 55mm lenses if you don't want the zoom, for about the same cost.
The contax is a modern electronic camera with AV/TV exposure modes as well as full manual operation, autobracketing, excellent viewfinder and readouts, mirror lock, and, most important, it was designed with digital backs in mind with all the contacts. Operation with the phase backs is totally seamless. Just place the back, select ISO and WB, and start shooting. It is not a camera full of multiple button presses and arcane menus--nothing gets in the way of you and your shot.
For its durability--pleass see my post of today in the medium format forum under the MF for landscape discussion.
With a $25K budget, this is a no-brainer!
Guy