I think the fact you are going to make it a usable space will make a huge difference. As for the sealing of the floor that is not an issue, thats why they have DPM :-). If you are worried about ventilation, stick some rads in, put some vents in the floor to equalise the moisture and the good OLD modern heating will sort the rest out. Forget plastering just caulk and tape the joints, a bit of sanding and your done. Insulating between the beams go for 50mm so you have useful joist still that you can hang things up in. I reckon you could get an X4 size machine down there no probs.