Echo the comments about what you require from the van, I have deliberated a lot about getting my tintop converted recently but always go full circle to why and what I wanted the Bongo to do for me, so I cannot justify £3K+ for headroom (I don't really care for) and storage (I don't need) the option of sleeping in the AFT definitely appeals but that is pretty much seasonal and for when there is 3 of us, which means using a site which I often find pretty unappealing (noise, charges, amount of people/vans) prefer ad-hoc/wild camping, then the erected AFT would alert one to the fact you are camping in the van. I purchased a drive-away awning for the extra bedroom.
My tintop is used as a day van for MTB rides and hiking trips with a comfortable place to eat, drink and maybe have a kip or read afterwards, with ad-hoc low key overnight wild camps (and trips to the dump as it goes under the barrier). If you are looking at predominately site camping the AFT would have my vote. It looks really well designed whilst some of the aftermarket pops seem to lack the ability of pushing the bed section out of the way(?) quickly when not needed; they seem to use bed boards which then again would limit the headroom when in position. The headroom will also be restricted if you are using the AFT to store items too I guess, so it then becomes akin to a tin top with more toot you don't need with you

(?). Throw into the mix of a camper van conversion, if I had a side conversion the AFT would definitely be needed given the small bed.
If the Bongo you are considering is more elderly then for £3-6K it seems to be the fitted to the majority, but then consider you may well have in this price bracket the tin worm problems and well documented mechanical issues to attend to. Private or Trade?
A newer facelift camper converted AFT is between £11-17K and is usually somewhere between £3-5K over and above a similar spec tin-top.
Or there is the option of a Roof tent?