Max Tyre dia for 97 Grand cherokee

[Craydiver]

My mate has brought a jeep to get into 4wding
97 Grand Cherokee Larado
Ive told him he just needs to get a good set of tyres and get out there first to try before heaps of $$ on mods

Talking to a few in club, My research for him indicates that he should fit 31 MT's or is that pushing it.?
Also what the best offset on the 15x7's on these?
I haven't been able to find any posts with definite answers on this.
If hes going to fork out $1500 on tyres and rims, we're keen to get this right, else I can see him then spending $ to lift this afterwards.
Hes willing to mod but ive told him step by step as he totally new to the offroading.

Im out of my league here being a saf driver