Without putting too much of a damper on your plans I think it will cost too much to set up, there will be a lot of work involved (I mean a lot) and if you are planning this alone then you are going to be spending a lot of your time creating and setting it all up ... won't happen overnight or even in a fortnight ... more like 6-9 months (less if you have help and possibly loadsa money ... are you Harry Enfield)
Seriously, I admire you if you go for it but you are going to find it difficult without some form of backing.
You will need your own server(s) as a lot of the web spaces you can buy will not allow that kind of bandwidth, also need to employ the programmers or pay for programmes you will need to use.
The other option is as a Reseller, a few places offer this option where most of the work is done for you ... problem is if they go down the tubes then your customers will come after you and you have nowhere to go.
Anyway, a few things there to think heavily on and apologies if I have put a damper on it all for you ... if you do continue then Good Luck with your venture.
