Exam wise, I don't think you would need to do more than the helicopter technical and the paper specific to the type so, two more exams at the most.
Flying wise (for a PPL(H)), you can only get a maximum of a 6 hour "allowance" for having a fixed wing licence so the minimum is 39 hours.
If you're looking at a Schweizer and you're in Dorset, we are thinking Bournemouth Helicopters. Give them a ring for prices. I have flown with them a few times and was more than happy with the tuition received. You will be looking at around £280 an hour inc VAT and landing fee.
It's unlikely that you would pass in minimum hours; few people do but, with a fixed wing licence already you may not be far off the mark.
I'm not sure I would recommend a PPL on a Hiller only because there are so few in the country but, if there is one near to you that you will be able to fly post-PPL, then that may be an option. But the Schweizer is a cracking little helicopter!
Cheers
Whirls