Stevo, what makes you think the frames need to be changed?
Can they really be that bad? Even if the frame is damaged at the hinge locations you can just position the new hinges slightly differently. Adding frames makes it a longer, dirtier more expensive job. I'd imagine the frames could possibly be rubbed down and re painted?
As for hanging doors, it's genuinely quite straightforward, there are loads of 'how to' videos on youtube and it doesn't need any crazy expensive specialist tools. Oh, and as learning a little bit of DIY goes - it's indoors in the warm, it's private so nobody sees if you cock it up, you're fairly unlikely to electrocute yourself or flood the house. It has to go really quite wrong for you to die, burn, be shocked or drown.
The tools you'll need are straightforward, cost much less than 2 hours of a carpenter's time - and you'll still have them for the next job that comes along.
The tools can even be bought from Ikea and Argos if you're shy about DIY stores.
New door size - measure the door leaf you've got - on the presumption it was the right size for the hole in the wall, go on the Wickes or Screwfix or Homebase website and see what style you like.
Not buying two frames could save you £50 or £60, not getting a chippy in could save you maybe, £40 or £50 an hour for 2 hours work. Potentially, you could buy yourself a neat little collection of tools, learn a bit of DIY and be the best part of a ton better off.