You've got your work cut out there..it's a bigger job than you think. As an independent HETAS engineer I often advise homeowners on how to do most of the building work them selves to keep the costs down, or use a builder because they are cheaper and quicker than myself. The previous comments were correct.
This is clearly an old unlined brick chimney with some lousy brick work...
It will leak dangerous CO fumes and smoke like a sieve without a new lining, and the lining will have to be a minimum of 200mm internal diameter, irrespective of what material you use. = bucks,
Any work on a chimney and fireplace requires Building Control approval, either directly or via a HETAS engineer. Unless the HETAS person does the work them selves don't expect them to certify it for you, too risky. It's not a matter of them making money, it is seriously high risk, CO from a fire or stove is many times that of a gas fire.
A 200mm liner does not like to go down a 200mm chimney, then you have to fit a stainless steel hood inside above the fireplace. Probably 80% of the heat from an open fire will go up the chimney, a total waste of time and effort and hard cash.
Your best option is to get yourself an independent HETAS engineer, not a stove shop, and go for a stove which takes a 5 or 6 inch flexible liner, leaving you just the fireplace and hearth to sort out (first). Find out and fix the dodgy brickwork at the back, make sure it is thick enough not to set fire to your neighbour or fall outside, and why is there that waterstainng on the sides. Exposed brickwork is ok , but if you want to render the inside it'll have to be lime cement render to prevent cracking, or cement boards, both work fine.
Lots of independent HETAS engineers will fit liner and new stoves (forget second-hand, most ain't legit no more) purchased by clients, provided, they have been involved frm the outset and it's the right sort/legit.
Your choice of stove will affect the hearth size and type, it looks as if you already have a basic constructional hearth underneath, so something non-combustible on top is all that it required, but of sufficient size - 150mm either side of the stove/opening, at least 225mm infront of the stove, 300 is better.
As a DIYNot website, I think I should commend you to read the Stovefitters Manual website, lots of good info on there. Hopew this is of help.
