This is such a wide ranging question that answers will just keep on coming.
Just to put it out there, we bought a NB as our retirement property a few years ago (not your developer) and are very happy.
Large, well designed & built house with high ceilings, plenty of room space, good sized gardens, plenty of private parking/garage, well laid out development with great walks plus easy reach to facilities.
Essentially it ticked all our boxes.
Firstly, ignore the "never buy a NB, they are all cardboard crap, built on a plot the size of a postage stamp, will never be worth what you paid for it" etc etc. These comments blind you to genuine, informed responses.
Research your developer, do they have a good reputation for build quality and after sales.
The development itself, does it look well laid out with plenty of parking, green spaces, proximity to services (schools, shops, medical, buses etc). How long will building be going on after you move in.
What about the plot, is it overlooked, have sufficient parking (NEVER rely on on-street parking, if you do so will everyone else), is the garden adequate for your needs.
What is the construction of the property, some are timber framed while others stick with brick/block and use block internal walls. If semi, are living /sleeping spaces against the party wall, what are noise transfer levels like.
Does the layout of the house work for you, does living space match the number of bedrooms.
Is there a management fee for green space maintenance (very common now and can be up to £200/pa).
Check thoroughly what is included as there will be an extensive options list and while some of it will be more costly than having it done yourself later, the ease of having it done ready for when you move in is priceless.
Check plans for position of sockets, lights, TV & ethernet points, outside lights & sockets, garden tap etc. Kitchen & bathroom upgrades can sometimes be worth it.
Are they throwing in any extras such as turf, white goods, flooring etc and what about help with solicitors, SD, removals etc.
Many NB gardens have issues in the first couple of years and even if turfed, may require work to help with drainage but these issues are usually resolved very easily and if you are keen gardeners (like us) you can do it yourself as you are planting.
If you have ANY problems with the house make sure you report them via the developers portal and if they are not resolved, KEEP REPORTING as they are obliged to fix these issues even after the original warranty has expired.
Loads more things will come to mind later.......😃