1. One that has Wireless Bridge (or AP Client) capability (not too many do).
2. Probably depends on how clear the instructions (in the User Guide) are. Usually easier if both routers are the same brand.
3. Depends. If the antenna is comparable to the one on your computer's adapter and you put it in roughly the same place I would expect "when decently connected average an unsteady 5mbs." That's assuming that your wireless adapter and the to-be-purchased router are both 802.11g or both 802.11n. You probably can expect better results if you are "upgrading" to a better antenna, from 'g' to 'n', or to a location in the room where the signal happens to be stronger.
2. Probably depends on how clear the instructions (in the User Guide) are. Usually easier if both routers are the same brand.
3. Depends. If the antenna is comparable to the one on your computer's adapter and you put it in roughly the same place I would expect "when decently connected average an unsteady 5mbs." That's assuming that your wireless adapter and the to-be-purchased router are both 802.11g or both 802.11n. You probably can expect better results if you are "upgrading" to a better antenna, from 'g' to 'n', or to a location in the room where the signal happens to be stronger.