1) Stryker - This is pretty large for 15mm, so suits med/large tank, would look good, wheeled, hover or grav.
2) Saracen - perfect for a medium APC, probably look best as wheeled or hover.
3) LAV - good size for a med/large APC, this would look great as a grav APC.
4) Scimitar - nice for a light tank, hover is the most likely choice for this.
5) sheridan - good for a med/large tank, again hover is the best choice.
6) Bradley - not sure I will use this for this force, maybe for the NI, grav or hover.
7) Merkava III - good for a heavy/super heavy tank, tracked would make it look to much like is donor kit, so hover or grav.
My real dilemma is whether to go hover, tracked, wheeled, or grav, here's my train of thought
1) Wheeled - for this force I would prefer wheeled, however, the Saracen and Stryker are already wheeled, while this seems to make the conversion easier, it actually will make it harder, because I don't want the final vehicle to resemble the original to closely.
2) Hover -this choice makes sense for a mid tech force, an easy way to have an all terrain vehicle without the tech level or price of Grav, and it will look scifi rather then modern.
3) Tracked - I think this would look very cool putting tracks on a Stryker and Saracen, but the reality is that it would be fun to make 1 of each, but I want at least half a dozen of each, and I would need 2 donor kits for the one vehicle, so would be expensive.
4) Grav really is to high tech for this force, but I think I could make them look really cool, I might save this for my NI force.
Have a look at the attached picts and let me know what you think, these are only very rough concepts and will change a lot.