depends which route you want to take, if your planning on buyin one already done then anythin from £1000 upwards but if your planning on droppin the lump in yourself then it can work out cheaper
i've seen mb6 crashed donor cars been picked up for £300+ then all you've got to do is buy the car and drop the lump in, there will be a few things extra that you'll need to do the swap but its prolly cheaper to do it yourself.
I've seen b16a2 pre-facelits go for a £1000 but with high mileage 120k+
havent really looked for already done b18's but the little few i have seen have been priced at £1800+ never seen one cheaper then that really.