I'm looking for a bit of advice please.

I'm seriously thinking of buying an old (foundations C15, upper parts variously added since, up until the 1940s, but all 1 brick thick solid walls) farmhouse which will need a complete new heating and DHW system installed.
The ground floor is arranged with all the rooms in a long line, running from a kitchen/diner of approx. 21m2 area through 3 other rooms of approx. 14m2 each. The whole house is about 3.5m 'thick' from front to back internally.
There's barn attached to the far end, and on one side some single-storey additions which serve as an outside store and a couple of side rooms. They are each of about 3m2 area.
There is secondary double glazing fitted to the windows and the roof has relatively recently been re-covered, with loft insulation and the inside of the felt sprayed with some kind of yellow foam.

The house is set in approx 2.5 acres of land, which has rather run wild, with plenty of trees and large bushes. There is also a small stream running across the property approx. 100m from the house.
The stream looks to have a speed of around walking pace, so 1.3m/s or around 3mph. It's about 5cm deep at the moment, and about 1m wide. There's about 60m or so running across the property.

Historically, the house has used a solid fuel boiler supplying a DHW cylinder and radiators in each room.
These have been supplemented by open fires in the two main living rooms.
The boiler is no longer working, and looks to be quite old anyway, so a complete new system will need to be installed.

I'm working to a fairly tight budget so I won't be able to take ALL of the measures available to save energy, but I'd like to do as much as I can.

My plan so far is as below, and I wold be very grateful if anyone could suggest anything better (probably!!) or less expensive, and also give some idea of the likely costs involved.

1) Find and seal as many gaps as possible! = £100
2) Insulate loft to current standards = £250
3) Excavate concrete floors and fit wet UFH, with tiles on top
Estimate cost for insulation, 4 zone system, cheap-ish floor tiles = £5000
4) Fit new heating and DHW system. Possibly utilise exisiting radiators on top floor, tied to UFH downstairs
Here's where I get a bit lost...
I'm not sure if the stream is up to supplying a Water source heat pump, or even how much it would cost to fit one?
I have ballpark figures for GSHP of between £8-12k. That's quite a variation, which end of that scale might I be at?
What sort of output am I likely to need? We're a family of 4, both adults working full time, 2 children under 5, also out weekdays.
Is it possible to have the heat recovery source so far from the house?
If I fit the above, will I need a booster for DHW?
Will there be times in winter when we can't keep warm or have a hot bath, and my wife will hit me over the head and wish we'd bought a “sensible” house?
5) I've also thought about ST, but the house runs slightly west of true E/W so might not be suitable for panels on the roof, and as it's so old, I'd rather not stress the roof too much if I can help it!
Don't really want to go for EWI, it's a character property.
IWI isn't really an option either, as if the rooms were much smaller, we wouldn't fit inside! (also, the internal walls are much thicker towards the bottom and there;s not a straight bit in the whole place...)
If you've got this far, thanks for bearing with me!

IMO your house is totally unsuited to UFH.

With a high heat loss house you need to have a fast acting high temperature heating system, whereas UFH is best suited to well insulated houses that can cope with a slow acting heating system.

If you are intent on UFH then go for a system where the pipes are set in insulation placed on top of the slab with a thin screed on top but personally I would use radiators throughout the house with a suitable high temp heat source.

Thanks all for the comments so far - looks like you might have saved me far more than the £6 it cost to join!
Ok, so I wouldn't do too much in the first year other than stop up any leaks I came across and insulate the loft to the max.
I would need to put in a new heating and DHW system right from the start though. I forgot to mention before that it's not on the gas grid, so what might be the next cheapest system to install/run; Oil, Electric, or something else?

Hi to help with heat loss is it possible to place an additional door adjacient to back and front doors?
To create an air - lock. To stop all your hot air escaping when an outside door is opened often to be replaced with cold air.

We fitted one in our hall 10 or so years ago and I still congratulate myself on what a good idea it was.

Doing the same at back of house is more of a problem.

Arny owner of registered superhome in Chester

Spend £12000+ on insulating everything everywhere, forget the underfloor heating, buy some jumpers and a log burner and plant some trees for fuel - willow or chestnut coppice. Get some prices for exterior insulation - I know you want to keep the original character but not the cold! There's a whole range of option.

cheers
Tom