My best purchase was definitely a new boiler even though the previous one was not that old and also a positive air exchange unit at the same time. I knew when I bought the house that the boiler was not big enough for the number of radiators and I had the electric fire removed so I could fit my furniture in. So I suffered with a number of damp problems for the first few winters in the bedrooms and hall. Also I struggled to get the lounge temperature to a comfortable level to relax in at night, visitors used to complain about the cold!
Since the boiler, togetger with an extra radiator, was fitted, I have been snug and warm with no trace of damp, greatly reduced condensation on the windows and a lovely fresh smell around the house. I think the positive air exchanger helped as well.
Worst purchase having new gutters and fascias fitted the year before a had a new roof, the gutters did not fit the new roof exactly which prevented the roofer from doing a neat job.