Re: stamp duty
I think that the following should work.
Agree the purchase price with the vendor.
Then do two contracts for sale.
You are buying one property as your principal private residence.
Your friend is separately buying 3 properties as an investment.
You will pay no stamp duty.
The vendor's solicitor will have to be paid to do the extra work and so will your solicitor, so make sure that the saving is worth it.
But it's probably worth doing anyway to have separate ownership of the properties and not to be partners.
Of course, you will need separate mortgages as well, which might be a problem. But your residential mortgage should be cheaper than his residential investment mortgage.
Brendan