Well replacing a boiler won't be cheap so I'm assuming that wasn't in the original plan.
Bathrooms are as cheap or expensive as you make them. My en-suite at the last house, I bought a 900mm quadrant glass shower with stone tray for £160 from Tesco bathrooms, you could buy pretty much the same thing from B&Q for about £800 and spend about £1500 from more "luxurious" bathroom places.
Taps can be ridiculous too.
My entire en-suite, so shower enclosure, electric shower, sink, taps, back to wall toilet in a cabinet, tiling in shower area and above the sink, mirror, flooring (Value range from B&Q) paint, and re-plastering cost IIRC £2000. Yes, we did fit it ourselves except an electrician connected up the shower.
It is easy to get seduced. Google like mad for products, I thought I would go crazy if I had to look at another sink but it was worth it in the end.