Just remember, the prices that boats sell for is usually not what they've been advertised for.
In fact in historic, or ex. work boats as you call them, i suspect the sale price is often considerably less than the price advertised and most hang around for sale for a while.
IMO the going average price for an unconverted boat is £45k ball park. The price for a converted boat £65k ball park. Of course there would be variations either way depending if boats were in exceptional condition (for example a new bottom), or in poor condition and needing significant work.
If you were thinking of converting a historic boat, then 'Otley' would be a very good option. It's already had some conversion done and 'seems' to be a very attractive price.
Personally (and I'm no expert) i think if you were to buy an unconverted boat and convert it and fit it out to any reasonable standard, i think you'd be looking at well over £20k, possibly double, and i doubt it would add that value to an asking price if you were doing to make money (short or long term).
The only way i could ever seeing your idea be viable was if you could do the work yourself.