You already know what i think of s2k engines! they are a masterpiece, feel a little underpowered in higher gears in the s2000 but 1st and 2nd are frantic
Couple that with half the weight in a roadster and it would be phenominal
2.5k is steep though, find one in a scrappy that doesnt do kit car packages etc and you will get all you need for less than that
You also have to factor in the cost of a decent aftermarket ecu as the standard one is meant to be extremely difficult to use when its not attached to all the gubbins of the s2k