First, welcome to the forum. the epmp is a solid product line and will do you well. for very dense situations, you may want to consider the 450 line as it is more scaleable. (the 450m can feed a few hundred subs with ease)
1. Best base station hardware, and backhual (including Router, full duplex managed switch, server, etc) for atleast 1500 subscribers at list 5 base station.
best basestation is going to vary a lot in many situations, LOS, nLOS,NLOS all play a factor. our distribution sites we run both 2.4 and 5ghz epmp and comfortably connect 100 to 250 subs per tower. the dual freq sectors from KP performance make life for us easy and keeps our tower leased down. they have very good beam performance. RF elements makes a horn sector that's awesome for filling capacity needs on towers. the cambium sectors have great signal characteristics
routers are always a hot top to debate. we use juniper at our core for heavy loads and light load stuff i've used the UBNT edge routers successfully. others prefer mikrotik end to end. low cost routers now have much more power than they ever have. definitely talk to tik guy if you want to head that way.
light backhauling we use EPMP. rough situations or very long shots PTP650 is great. short high capacity PTP i'll use AF24 from ubnt. anything critical, high noise and high capacity and range the PTP 820 series is a great choice.
2. I want to offer upto 4-5mbps connection to subscribers.
thats a good start for offering. the epmp line can comfortably offer more. we offer up to 15 mbps on the 2.4 nlos and 50 mbps on the 5ghz.
3. What type of sector antennas should i use 2.4ghz or 5ghz.
the cambium sectors work wonderfully. the KP dual freq sectors work just as well and will let you combine into a single basestation.
4. i will use epmp 1000 Gps and force 180
GPS definitely. the force 200 is a great antenna as well, id recommend switching to them any time the 180 isn't running at MCS15 to keep your network delivering as fast as possible.
5. I will employ an Network Engineer (operator) to do all the technical work.
great idea, that will save you a LOT of headache.
6. There is no interference in the air (no other wisp)
there will always be interference. wireless routers will cause a dull roar you'll have to deal with. we try as much as possible to deploy R200 or R201 cnpilot router and remote manage them for the customers so we can help control the noise and keep everything clean.