There is also the complexity of single vs multi engine IRs. If I were to do the IR on an SEP, how could I upgrade the IR to be valid for ME flights if were to get an MEP class rating in the future?
That's easier. 5 hours instrument flying on the MEP and a skills test.
However, since most of a full IR - in the order of 30 hours - is in a simulator, whether SEP or MEP, the difference in price becomes much smaller, since the sim costs about as much as an SEP, and the price difference between a complex SEP and an MEP is only around £150 per hour.