If your chimney is ok then a s/steel flexi liner would be my recommendation, then infill between the flue and chimney with vermiculite.
You will need to work out the square meterage of the room, this will allow you to work out what output you need from the stove. There's no point in getting one that either underheats or overheats the room. If you don't have insulated walls or double glazing you might want to adjust your figures a bit.
If you want multi fuel make sure the stove is suitable, some are only ok for wood.
Do you want a back boiler? this would allow you to save a few bob on oil costs as you would heat your water when the stove is on. This would depend on the set up you have now of course.
Personally I would be wary of the cheaper models, some are fine but others, well, they might be more trouble than there worth.
At present we have 2 one of these........
http://www.morsoe.co.uk/brændeovn-3.aspx?M=eCom_Catalog&PID=&ProductID=PROD173 and one of these, the G2 here.........
http://www.fireplaces-oldham.co.uk/Austrofl.htm
If I was going to replace one or get another or may be one for the summer house I like the Windy Smithy ones............
http://www.windysmithy.co.uk/html/woodburners.htm
There's loads of choice out there so it looks like you have a few hours/days to research