The most important at this stage is to obtain as many guarantees as possible in writing:
- from the academic
- from the head of department
- from the local graduate school
Essentially, academics are meant to dedicate a proportion of their employed time for supervisory duties, so that all academics have supervision workload fairly spread across a whole department, also variegated depending on the role of the academics (lecturer, reader, professor).
If the academic is employed in the UK higher education institution for 50% full-time equivalent, then their supervisory duty should be halved by comparison with colleagues.
If he said that you will be 'that I would be the only PhD student he will be supervising during that time', that's probably what's behind the statement.
However, it does no harm to have this employment arrangement confirmed by the head of department and the local graduate school who ultimately have to agree to such arrangement. So you can make sure the academic understood well.
As Captain Emacs points out, then, there is a lot that boils down to your own expectations and needs in terms of supervision. There are bound to be pros and cons. Simple basic question: is the academic very IT-literate or does he do everything from print-outs? If he's constantly travelling between job places, that might become inconvenient when drafts need careful attention in a reasonable turnaround period.
The more you discuss and nails the practicalities of the relationship at an early stage, the less likely it is going to go bad.
Last but not least, if you get funded with a local scholarship or a grant from one of the big funding councils and charities, your supervision is more likely to be successful, because the funding set up will create an additional level of accountability for both you and supervisor.
