What country are you in? 900Mhz is not allowed in most European countries. Other parts of the world I don´t know.
What region do you want to setup? Urbanised area (city, estate?). In cities you might find problems with interferences due lots of other 2,4ghz users. I would suggest 5Ghz. Less interferences and also less problems with hackers probably. Not a lot of 5Ghz stuff to be found in the white good shop around the corner.
My AP´s are rb532A (older units, not available anymore) and its successor, the rb333's. As CPE I use mainly rb133c's and some old rb112's.
My main router is a Pentium PC running ros. I am planning to replace the latter with a rb1000 sooner or later.
The advantage of an all MT network is that lots of configs are arranged easier then working with a mixture of brands. Plus you only have to learn how the MT works!
Also, if both close proximate CPE's and distant CPE's are to be using same AP adjust the power levels of the CPE radio cards in such way the power readings of the CPE's in the AP are all in relative same levels. Thus decreasing power output for short distant units, no change for medium range and only minor increases for the distant units. (Never go for full power! It will burn the radios. Also, lots of connectivity problems are due to high power outputs. Very often it is better to reduce some power, even if you initially would think of increasing it.)
Depending on traffic, the amount of firewall filters, mangle and queuing rules you should calculate on a setup of 30/40 to max 50/60 CPE's per AP.
My network is fully routed where clients are always on line, making part of the network. If you understand the principle it´s fast, reliable and not a lot of config needed. But some want client authentication and then other methods might work better.
Rudy