Unsure whether to use "enquire" or "inquire"
Almost certainly OP should be writing to enquire, since he's presumably writing to ask about something, but not in the context of an official inquiry.
To the extent that there are two different words (and, frankly, many people don't distinguish), inquire conveys more a sense of formal, official investigation.
It's unlikely OP's correspondent would make that distinction and be alarmed if he wrote inquire (or indeed, be concerned about OP's vocabulary), but better safe than sorry – if in doubt, use enquire.