Single engine IFR is prohibited pretty much anywhere except for OZ and CA for public transport. But certainly the UK/US regard instructing as a separate case, and do permit it.
Sounds to me like you're specifying something like a big-engined Cherokee variant with the outboard tanks option, and ideally anti-icing. A Lance or Saratoga perhaps? Likely to cost you around £70-90k. I believe that anti-icing kits are available for these.
You'd have to be flying silly (500+ pa) hours in an airframe to justify turbine costs, and I certainly doubt you'd get anything with a turbine for under £100k.
G