I'm currently in the process of preparing my mantle piece and fire surround for fitment of a wood burner too and I can say that everything you have just read is true.
I have heard many reports as to what you can and can't do and in the end I decided to read the building regulations regarding solid fuel burners.
Yes, as with most things now building regulations need to be adhered to with the fitment which means you need to allow clearance around the appliance, minimum 100mm I recall and this must be a non combustable material. You need a stainless steel flue even if you currently have a chimney and this is the expensive bit
Our house was built in the 30s so has a 9 inch square chimney and we were incorrectly told that this would be sufficient if we had it pressure tested and swept but apparently not!
Wood burners are very efficient so the gases passing up the chimney are cooler than those of an inefficient open fire. (More heat going into the room with a burner). This means that the chimney internal walls can build up condensation and any particles passing up the chimney stick to them thereby creating a potential fire hazard if a stray ember were to blow up there. Result......chimney fire:yikes:
If you talk to any fireman, they will tell you that most chimney fires start because of this. Even with a stainless flue you still need to have it swept annually so this argument doesn't hold out either...........I asked
In terms of ventilation the rules are that you need a direct source of fresh air into a room if the burner is above 5KW and this doesn't include trickle vents in windows or poorly fitting doors, it needs to be a dedicated path from the outside.
I don't want to put anyone off doing this but I just thought I'd fill you in from our own experience and yes we're still going ahead with it but we are doing it properly.
Budgetting wise I would recommend you multiply the cost of the burner by 2-3 times for the installation depending on how long your flue needs to be and how much you think you can do yourself. An approved Hetas fitter will also give you an installation certificate which you will need if you ever want to sell your house.