Cost of a cellar conversion

I'm looking at buying a house and making the cellar into a big family room. It's very dry and has a good head height. It has a shower room down there already with a loo and a sink (so plumbing and power are there). There's also a utility room there with a wm and dryer. There are stairs to it from the kitchen and there is a door (to the outside decking area) which is not full height - think it is about 5 foot (which we plan to replace with patio doors and lower the deck (tbc)).

I think we would need to just tank it, insulate it, run heating to it, put flooring and plasterboard / plastering up, put in a door and adjust the outside level so the door can be full size (and decorate). But I've never done this before so not sure.

How much would this cost? What kind of ball park?

We are a few weeks away from moving in and I want to be able to budget for it now so we can do it asap when we move.