hi. do you think this is a fair swap, my 2005 slk 350 66k on sale ebay http://www.ebay.co.uk/itm/Mercedes-SLK350-3-5-7G-auto-66k-Convertible-05-p-x-why-audi-tt-coupe-bmw-z4-z3-/221256112930?pt=Automobiles_UK&hash=item3383e45b22 for a 2006 z4 2.5 si 61k ebay http://www.ebay.co.uk/itm/261254786657?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649 thanks in advance. 