First of all you need to decide what you intend to use it for, how much time you intend to spend on learning how to shoot it well and what distances you want to compete at.
You should spend some time at Bisley or other ranges and see what is being used and take advice from club members.
I use a .577 at short ranges, 100 & 200 yards and a Parker Hale 0.451 Volunteer upto 300 yards but I know some of the club members will take the same rifles to 500 & 600 yards.

I have a Parker Hale 2 band Volunteer for sale, posted 27 May.

If you want something a bit different I also have a Scheutzen 0.45 rifle that can be used from 50 yards to 200 yards. This is in the Perdersoli catalogue and is rather unusual as the hammer travels towards you! It is safe with an eye blinder and it has an integral shield. Not something for novices!
HM