Hmmm, pretty mcch every 29er will take a rear rack (maybe with P-clips and a little bodging) I guess price range would help but the Haro Mary is quite a nice bike for the £££ (either geared or s/s) or you could look at building up a frame although that will cost a little more and helps if you have a definite “plan” for the spec. Surly, Salsa, On-One, Singular and others all make nice frames
I would say that its important to get the fit right for longer trips and if you go for an off the peg bike look at switching out saddles, pedals, grips etc as well as getting the bars / saddle / pedals in the right places.
The 29er forum on MTBR.com is a good place to get info on brands and info on specific stuff like rims / forks etc…
SSP