Not technically true that in every case a condensing boiler must be fitted, there is an exemption criteria whereby some installations can still have a non condenser if it meets the points total
Probably won't be the case here but it could be, however a more serious issue would be taking responsibility for a second hand appliance, without knowing the condition of it,
There is a massive market in the Glasgow area for brand new non condensers, with a well know chain reporting that they sell many 100's every month, I know they won't all be going in places which meet the criteria but at least they are new and it's keeping the cost down for the customers, I believe you can get a Vokera standard efficiency boiler, flue and mech clock for under £450