I agree the conversion is cheaper but i would save and spend 6K on an old elise

2.5 - 3k conversion
1.5 k mk1

4 - 4.5 k conversion mk1 - probably more expensive to insure than a std elise, maybe unreliable if compared to some (not yours) mk1.5's
plus ££ for suspension upgrades £400 and brake upgrades £? 200-400

=~£5k

You asked for positive and neg comments:

I like the conversion idea in principle - good engine and nicely suited to the mk1, but value for money, i'd save a bit more and buy something off the shelf