You might want to consider going straight to the Pitts (or a share in one). Buying and selling aircraft always takes time and expense. Its best to get one that you won't grow out of too soon. All the other aircraft mentioned will do aerobatics but are quite limited in aerobatic performance.
Despite being a confirmed monoplane pilot I would recomend a single seat Pitts as a great aerobatic starter machine. Excellent value for money, slghtly tricky to land. You will need to get a tailwheel sign off and probably 5 hrs dual in a 2 seat Pitts.
There are always some for sale on the BAeA wbsite.
http://www.aerobatics.org.uk/aircraft+eqpt-for-sale.htm
Good luck