If you are not good at woodwork, ask around for a recommended carpenter/joiner.
It is easy to do it wrong, and you will be looking at the results for a long time. Fitting the lock(s) also has to be done neatly and accurately or you will weaken it
edited - sorry I thought you meant a new door, not a frame. But I think my reply is still true - the door will have to be fitted to the new frame. Buy a hardwood one. Lift a few up to get the feel and avoid the flimsy light ones. If your old one rotted, give it a few coats of Cuprinol Clear and let it dry for a week before painting the back and having it fitted.
I fitted my hardwood front door, and got very tired lifting it up and down for trial fits (I later put it on the bathroom scales and found it weighed 40kg). I learned from that and got a local joiner in to to the back doors. He was also a lot quicker than me.
Think about having a new hardwood door-frame as well, unless your old one is solid and in good condition.
I found it an advantage to apply preservative and stain to the doors and frames before fitting - much easier to do the edges that will be against the wall and floor afterwards.
Your carpenter will probably cut the edges off the door when fitting, and may scuff it, so don't be disappointed if you have to touch it up aftwards.
Have a good mortice lock fitted as well as a (spit) nightlatch. If you can't afford a Chubb there are some quite good BS locks sold under own-label brands. If you get a standard size it will be no trouble to change later it if you lose a key.
The time to fit will vary depending on how straight your walls are. Tell the fitter what kind of locks, knobs and letterbox you want and ask if he minds you buying them himself.
Have the letterbox well away from the latch - if you have a panel beside the door, put it in there. There are ways of manipulating a lock through a letterbox to open the door very easily.
p.s. Please don't buy a plastic door.
