We had this issue when we applied for the Hampshire Trading Standards 'Buy With Confidence Scheme'.
We didn't do PDI's and they told us it'd be a good idea in-case we are asked to prove that a car did not have a pre-existing fault at point of sale, to which we agreed, but I told them I wasn't a trained mechanic, so whilst happy to fill it in, didn't want to be fraudulently saying something's fine when I hadn't ligitimately checked it!
The answer was pretty much 'do as many checks as you can as it's better than nothing'. The road test sections will throw up a lot of items, but as you say, there's no substitute for getting it on the ramp to check Suspension/Brake items/leaks etc. Not comfortable with 'do as many checks as you can as it's better than nothing', we ended up MOT'ing pretty much all of our cars which allowed us to fill in the convenience items ourselves (leccy windows etc) and the MOT would allow us to complete the majority of the other stuff, having said that, there is still some obscure items on there such as 'pressure test radiator cap' amongst others, we simply mark 'nc' not checked on these items. Not ideal, as this may raise questions in itself but I guess the only option here is to create a custom PDI form, maybe a simple one in the format of main dealers vehicle health checks? Can anyone recommend a custom print shop who can offer this and one they use?
anyway, we now have a mechanic that carries out PDI's on all of our cars, in addition to a new MOT if it has less than 6 months remaining.