1. You need AS (Autonomus System) number for use EBGP. You have to apply request to ripe or arin NET for to get public AS number.
2. you can use if your ISP allow this IP. You will start to anounce yourself which IP is working with you. Also you may request your IP addres from Arin or Ripe network. (it will be better. If you need to change ISP, your IP address never change)
3. You can. But Load balance on BGP needs qualifed engineering ability. You can not learn in here how to do it easly. You need to monitor and according to request load you can change traffic direction of packets.
4. Both of them possible. 1. topology will provide redundancy but load balance is more complicated. 2. topology more easy. Just you are losing hardware redundancy. If you have stable electrcity infrostructure in system room i recommend 2. topology. Cheaper and easy then first one. you may change in future if you need redundancy.
you can use Cisco2821 with 1GB Ram for BGP session. It is really enough for your operation. I said 1 GB RAM, because you need full anounce of networks for load balance operation. If you dont need load balance you can get just default route info. This time you dont need RAM.
