Welcome Along
Very nice motors you have/had. As other have said its a shame you have had a poor experience with the Merc.
Going from that to a 370, I think your would miss the noise and acceleration of the Merc, (new exhaust will solve one of those) but more refined than the S2000. Which if your looking for something in the middle, then the 370 may just fit.
16.5K for a 59 plate..... I would say thats a good deal, especially as you know the history of the car
Have a look on the US forums there are loads of ICE builds so I don't think its a difficult job to change the HU.