If the flue off the stove is 7" then it has to be 7" liner, no if's, no but's, no maybe's.
As for grade of liner, go for the best, 904/904.
316/316 is for smokeless fuels and will burn thru in no time if your burning wood.
904/316 is better for wood, but as its in an existing chimney which will have acidic deposits from previous fires this too will probably burn thru from the outside in.
As for having 2 HETAS guys coming out quoting you must use a 6" liner, then I would actually question thier experience.
Dont forget as you are more or less re-installing the appliance it should comply with current regulations so you need to take ventilation into account, the hearth, a co alarm and also registering the flue and/or stove installation with LABC.