I think Bongos are fine, had one since 2005. As long as you fit a coolant alarm (£50-60) then the overheating issues can be overcome. The mileage is like anything, the faster you go, the worse they are. Cruising at 60mph which is pleasant gets me 30 + mpg on diesel. Having the kick down boost on acceleration will get you out of any trouble on a motorway, it can fly at 90mph should you desire. The best bit is they fit through 6'6" skinny residential barriers. We had a rocknroll bed in our 1st, and in our 2nd (1st was written off by a reversing lorry!), we just put the seats down and a double mattress sized bit of foam. V comfortable. 1st had a pop top roof which was great for our 2 kids up to the age of about 8, then we just got a tin top after Bongo 1 was written off and attach an awning with 2 campbeds each side. So sleeps 4 comfortably with an awning. What's nice is the things like the fitted blinds, we fitted a leisure battery and split relay so it charges when driving and that lasts all weekend. Under the bonnet there is a space for a chunky leisure battery. Insurance is cheap enough £3-400 per year. Good all rounders for a 1st camper. A poor person's VeeDub.
We paid 5k for a 1996/7 in 2005, and then 5k again for a tin top in 2016. The good thing is that they tend to have low mileage, check the speedo isn't in KM rather than miles, ours was a Japanese import with 100,000km on the clock, so north of 60,000 miles, which for a diesel is great.