If you decided on a 4/4 the new 3.7 litre Roadster is probably too much for you. I was thinking of the 3 litre model but it was discontinued I decided to opt for a Plus 4 with a whole lot of options.
You may find that for you a Plus 4 Sport fitted with a Librands exhaust system gives the ideal compromise between enough performance and what your pocket can justify, not only in purchase price but all the ongoing costs, tax, insurance etc.
Of course you could always slip a good second hand Mazda 2 litre engine into your existing car. Probably would not cost any more than changing over.