Something big like a UTV you would want the capability of being 4x4. As far as an engine, it depends on what you want to do with it. I've seen 1000cc motorcycle engines in homemade UTVs, some with super or turbochargers, and they hauled Arse. The 700 could be enough, but I am afraid you'd want more. Plus, you would really have to upgrade the clutch big time. You could find a nice V-twin, like a 95Ci for prolly 500 to a grand, and that wouldn't only sound mean, but it'd give you plenty of torque down low which you would need in something so heavy.
2008 700xx; Power Bomb header with power core 4 exhaust, Power Programmer III, pro armor nerfs, prm products control front and rear bumpers and full skid, 10" ITP rears, ITP Baja's fronts, RZR 2's all the way around, Uni filter, 13t sprocket, CCP steering Damper, DS450 shock swap, Dual Gasser Throttle, bar risers and Moose barkBusters. 730XX kit plus other engine mods!