The biggest issue is that they aren't £15-20k (unless you find one that is already built) - they end up being a large chunk more because few people 'just' do the engine.
If you speak with the owner of Rocketeer, he'll tell you that most of the built cars (rather than those people who buy the engine kit and self install) end up with suspension upgrades, interiors, etc. Some of them end up £50k and high £20k to £30k is more normal.
The other downside is resale - they don't command strong money on the secondary market. I attribute this to Rocketeer getting some average press early on and lacklustre marketing since.
I like the Mk1 MX-5 a lot - they are composed, huge amounts of fun and well built. Whilst the Jag V6 swap may sound appealing, there are some excellent companies getting more power out of modifying the stock engine for a lot less money. The doesn't mean I don't like what Rocketeer are trying to do but simply that at the price point they are selling cars at, you have to really want a modified MX-5 over many other excellent candidates - a used Boxster or an Elise being just two.
