In addition to what Matt offered, I'd suggest you install and play around with LinkPlanner - try different radios from your prospective tower site(s) to various locations you would want to serve and see what it predicts...
We have ePMP (5GHz and a few 2.4GHz) with all 3 generations (ePMP1000,2000,3000) in the field, plus PMP450 in 900MHz band and older tech (PMP320) in 3GHz band. We're preparing to migrate from the older 3GHz to PMP450m 3GHz. (before anybody jumps in to scold us, we have an FCC-issued document saying we are covered to continue using old 3.65GHz tech until, IIRC, 2022)
I agree with Matt's summary that the 3GHz is probably your sweet spot. In the 2.4GHz band there's just far too much noise from everybody's wifi routers and what-not, making that a difficult band to deploy reliably. We're starting to see that more now in 5GHz, as well as more competing WISP tower sites.
900MHz is the greatest for things like aiming an SM straight into a tree trunk and getting workable connection, but the band is too small to satisfy the (modern, netflix-and-drink-while-sheltering-at-home) needs of a couple dozen or more clients. And depending on your location/region, you very well might find utility meters are already running in the band, and are likely spread-spectrum across the WHOLE band, leaving NO available channel that will not experience periodic crippling interference.
On the cost side of things, ePMP (2.4 and 5GHz) will be the least expensive, but the PMP450 product line is seriously robust equipment, both physically and capability-wise. A 5GHz ePMP sector with a dozen customers will cost you several times less than a PMP450M sector for the same customers, but if you need to push the capacity down the line, the ePMP will max out while PMP450 still has plenty of headroom.
I will differ with/elaborate upon one comment of Matt's - 5GHz works great in hilly/wooded areas... PROVIDED you install it above the trees... (I know, what a smartass) We are often installing 5GHz SMs on a PVC pipe sticking up 5-8ft above a treetop, some of our SMs are near 100ft above ground. Just sayin, you might not want to/be able to do the same.
Depending on your local factors, you may be well-served by approaching things from a micro-pop direction - drop a pole or short tower in the middle of a neighborhood, with one or more 5GHz APs (omni or standard sectors) and feed a dozen homes/businesses from that site, backhauled to your main tower sites. Particularly useful technique if you have 5-10 homes down in a valley (down the holler) with no possible LoS to existing towers. There's probably as many approaches as there are WISPs, but you'll find one that will work well for you. Welcome to the Cambium community.