Completely new to heat pumps

Reed_Richards

You are right to be concerned about the lack of insulation in your new house to be.  That will make a house more expensive to heat whatever heating system you use.  

The fact that the house has massive radiators is a good sign as that might indicate that they have been right-sized to allow the heat pump to run economically with relatively low output water temperatures.

I replaced an oil boiler and radiators with a heat pump and new radiators with fins to give them a larger effective surface area.  It has been fine since I did this at the end of 2020.  For a while the heat pump was a bit cheaper to run than the oil boiler but with the big rise in electricity prices it's a bit more expensive at present.  Quite a few people posting on this forum now have heat pumps, all the regulars seem happy with theirs.

Ask to see a copy of the MCS certificate and other certification for the heat pump in your new house.  There should be an MCS Certificate, an MCS Compliance Certificate, a Building Regulations Compliance Certificate, a Domestic Electrical Installation Certificate, a Commissioning Certificate (which gives the warranty details) and there may be an Insurance-Backed Guarantee.   

gm0

Big radiators are a good sign

If doing 2nd viewing or already checked.  Look at windows, doors, loft insulation depth.  Has internal or external wall insulation or a cavity fill been done.  Not all insulation upgrades are sensible on all older buildings.  Walls may not co-operate with cavity fill.  Or some existing room heights/sizes make internal sheet insulation less attractive than it might be elsewhere. Or planning lose it over a proposed external appearance change. 

There are good reasons why some upgrades don't happen.   And a characterful building is kept.  And compromises happen including running cost to do so on renewables. Or it is demolished to build new and better.

Taken together they tell you how much load this heat pump will need to carry.  Did the overall project skip insulation to avoid disturbing already done decor.  Was a heatpump fitted. And then radiator upgrades (because it did not work) Or designed in together.   Were some or all of the very sensible demand reduction things done. 
Understanding and sympathetic to the overall building and structure.

If the house HAS had insulation upgrades and the radiator upgrades - there is very little to fear other than learning to drive it optimally.   But if not.  Be a bit more careful.  Ask to see some bills and consumption data.  

The wrong size heat pump in a higher heat loss house can be an expensive business. 
Especially if it is then run too hot - to maintain comfort levels at peak. 

Electrical efficiency drops from the wanted 3x-4x per kWh in - with increased water temperature.
It can also fade in very cold and very damp conditions (defrost reversing) as they literally ice up and have to sort that out before continuing on a cycle.  Nature of the beast.   A feature not a problem.  It doesn't matter
If the system is designed correctly in the first place.

A post upgrade EPC will have some useful info on it but these are approximate to be charitable.

I would not buy a new house now without one of these designed in properly holistically.
So that I don't have to do the project.  

For a retrofit house - I would look at it as above. Was it done properly.  Did they skip expensive and invasive upgrades.  And if it is compromised.  Is it a compromise I am prepared to live with.