A carpenter can hang doors better and faster than a handyman or general builder. Find someone by personal recommendation, not from an advertising website masquerading as a recommendations site.
examine your frames for condition and square. modern door linings are not expensive, and can be quicker and give better results than trying to fit a new door into a wonky lining.
If you want a low-quality door that will look OK in the Estate Agents photos, you can buy a modern flush or pressed door. These are made of air with a thin decorative skin of ply or hardcoard and are complete rubbish, they can be bought around £30 upwards.
A solid-core paint-grade door is a decent item.
Veneered doors cost more, or are lower quality.
You can get good quality knobs, handles and hinges from here or buy the same articles at higher prices from boutique retailers and DIY outlets.
Decide early if you want brass, chrome, or brushed door furniture, and match it all.
Your homebuyer may well rip them out if they don't share your taste.
If you have, or need, fire doors, you will need something quite different.