Its definately something for the DIY'er. Firstly, remember - you wont have construction directions, so a bit of patience and planning is a must. It's so easy to get carried away with the dismantling and be left with a heap of bits and a lot of head scratching!
I'd suggest you take a few photos, close-ups of joints etc. Measure the existing dimensions while the GH is still up, to help you make a base/plinth accurately. An obvious starting point is to unclip and remove the glass, may be an idea to mark the glass with felt tip pen. Horticultural glass is usually 3mm and can become brittle, so take care and dont choose a very cold day! If you have access to a trailer, van or truck, then you will save oodles of time as you will be able to remove the roof bearers, release the ends and sides and transport then in one piece. Otherwise its complete dismantle and marking up of the bits. You will almost certainly break some glass and will also need some more glazing clips, so check a local glass/hardwear supplier. Good luck - I had a 12 x 10 offered me last week where we have just completed a job - I've just persuaded myself to go for it!!