Allan - Where are you located? It may be better to have one of our regional sales managers discuss your options. We always love to hear interest in our products, and want to do what we can to support testing to validate that it'll work for you.
You can email me directly if you'd rather not post publicly.
I posted some of my results with the 900PMP 450I in this thread. Look towards the bottom. Keep in mind this is an area where no other 900MHz solution would even stay connected. The PMP is passing 20Mbps down consistently.
There a a few other threads showing results just like mine as well.
I’m up river from you on the Iowa side about 2 hours I’ve been deploying mini POPs with very similar terrain that you’re dealing with. How did you deploy 3 years ago? single tower structure was it in Keokuk?
Depending on how many AP's/SM's you might deploy and could commit to, you might be able to convince a Cambium Regional Technical Manager (RTM) to bring out a test kit of PMP450i 900MHz. Alternatively, you could work with a major distributor, like Winncom, to do a test setup and/or try and buy.
In the grand scheme of things, spending about $3000 total for a test setup comprised of an AP, sector, and SM and yagi isn't that big an investment, and if this is 'breaking the bank' then perhaps you should re-evaluate your goals and finances before starting up another WISP.
If for some reason, you weren't able to accomplish any of the above ideas, worst case scenario, you buy it, test it, and if for some reason it doesn't work to your satisfaction... flip it on eBay or to another WISP and get almost all your money back.