There are some good Porsche specialists around - but the same rules apply no matter which specialist you use for which marque. You need to do your homework and speak to previous customers and see how happy they were (in accordance with their expectations). If you're looking for someone that can provide concours level restoration work, then track down an award winning 993 owner and find out which garaged they used. Porsche itself has a classic program, or you could try Porsche Classic Life at Bicester, which is supported by Dick Lovett. Neil Bainbridge has some award-winning cars under his belt, too.