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I would pay less for an Alpina touring than same condition M5, so use that as a benchmark.
If it just needs to be a V8 touring, then the 540 should suffice.
Realistically though, are you really going to sell the Volvo for maybe 50% of the purchase price of an unknown car that you potentially have to spend loads of money on anyway?
If you want the same peace of mind, you're going to have to spend £15k from one of those collector aimed dealerships for a proper minter.
That place seems, ahem, rather expensive. £100k for a 12 year old Honda NSX? And I paid the same as this (near as dammit (pun entirely intentional)):
http://www.millenniumheroes.com/cars/audi-rs6-avant-for-sale/
for my C6 RS6 Avant. Does anyone actually pay those prices?