Probably easier to mod your current wrx. By the time you search around, pay stamp duty etc, will it be really worth it? Sure theres probably a few decent MY99's going around with good mods, but more hassles!

HBF / RAC will cover mods to a certain extent. I know some ppl have insured ecu's et al with them, but a LOT havent. Just Car's are great with premiums and mods, but not so good with the excesses..