15x7 is the right rim for 235/75R15s. The advice to consider wider rims if there's a probability you'll go up in tire size later is wise. BUT ...
Also consider that buying aftermarket rims may not be a wise decision at all. If you're buying 235 tires, I'll assume that you probably have a stock XJ or maybe a budget boost, with fenders and flares intact. The stock, OEM rims are 15x7 with a 5-1/4" backspacing. There are NO aftermarket rims available, 15x7 or 15x8, that have that backspacing. Therefore, ANY aftermarket rims you get will result in the tires sticking farther out of the flares, and 15x8s will be even worse than 15x7s. It may or may not be enough to get you a ticket, depending on the rules where you live and how strictly they are enforced, but it will almost certainly guarantee that your tires will hit the flares when turning.
With 235s, I'd recommend staying on OEM rims, and wait to buy aftermarket wheels until you move up to larger tires that require a bigger wheel and less backspacing.