I did something similar when we bought our home 3yrs ago. I think it was about 10k, including switch to gas from oil. We found it really hard to get reasonable quotations from competent guys. Lots of cowboys out there. I'd lean towards paying a little extra if you can get someone who a friend or neighbour used and was happy with.
Also make sure you install a properly zoned system i.e. upstairs heat, downstairs heat, hot water, with wall stats in the hallway and master bedroom, and TRVs in all rooms except those with wall stats.
Seeing as you're gonna be lifting floorboards and re-plastering walls, what condition is your electrical wiring in? It will obviously add significantly to your costs but might save you going at the walls and floors ever again.