I've just done this and my previous experience was a few pipework alterations, new radiator fit and use of pre-soldered joints.
The first step was to do some research and plan the pipework. I replaced a single pipe, open vented, system (loop under each floor), with a dual pipe system that supplied downstairs with pipe drops (concrete floors downstairs). I had the boiler changed to a combi boiler, as a result the system was very simple to design and understand.
Next was to get an RGI round to make sure he was happy to install and commission the boiler if I did the rest. He advised I should use hep20 under the floorboards and copper for exposed pipework, so I did. All the soldering work was in accessible places, so it wasn't that difficult to make a solid joint with end feed fittings.
I was nowhere near as quick as a professional, but I believe I was more careful seeing as time wasn't much of a factor.
As for costs, it was cheaper, assuming I value my time as free - which I do as overall I enjoyed doing it. You'll save the day/hourly rate of paying someone else to do the work, but you can probably source some materials cheaper as well. All the professionals I know don't get time to scour ebay (or online merchants) looking for bargains and buy from builders merchants at trade prices instead.
As I see it, you've got to start somewhere. The worst you can do is make a complete hash and have to pay someone else to fix it, but you'll have learnt so much even doing that.
If you make a plan of your new system some of these kind cyber plumbers may even correct any potential mistakes