Hi and welcome/congratulations on the Bongo purchase.
There may be a few diesel/petrol specifics to look out for. Most below apply to both fuel variants. (I have a diesel)
Check the transmission fluid every now and then.
Check and replace the air filter regularly. (You can just turn them the first time you see them looking
a bit sooty) but probably replace once a year nominally.
Change diesel fuel filter every 2-3 years depending on mileage.
Fit a low coolant alarm and a temperature sensing alarm (I use just the temp alarm). If you were not aware aging Bongo coolant systems are prone to leaks, small and large. One must be aware of high engine temperatures ASAP with a coolant system fault. (the dash temperature meter is useless and by the time you see there is a problem there will likely be engine damage) Haydn alarms is the place to go for those. They cost a bit but it is pretty much nigh on essential to have at least 1 fitted for any Bongo. It is about £2K to replace a cracked head and have associated problem parts replaced. So worth every penny.
The diesel needs its offical oil change at every 6 months or 6K miles. People tend to have different opinions on this given the improvements in oil quality since the 15-20 years since Bongos were made. Recently I left mine 12 months but only di approx 4K miles. I plan every 9 months to do oil now just to be safe. You can do the oil filter every 12K miles. But I now plan every 9 months when I do my oil. (Did mine today in fact)
Check your drain hole under the windscreen every few months especially in winter they tend to block up and then water pours down into the fuse box in the front area. So check it regularly and clear detritus out of that little gutter/the drain hole.
If you have an auto free top remember to raise the roof around once a month to keep it lubricated/functioning (especially in winter when you tend not to use the Bongo as much) . They can have a habit of seixing up if you get unlucky so raising them up once a month seems on balance to be worthwhile. Sometimes you have to de-mould the tent in the roof if you have an auto free top post winter. (I like others fling some Silica Gel bags up there in the winter, seem to help a lot)
In winter periodically drive the Bongo and on decent 20mile + runs, once every couple of weeks at minimum to heat everything up, dispell moisture and keep things moving. Also run the air con to keep the lubricant in the system flowing round, that keeps the seals in good shape. They can dry out and crack and running the aircon for a hwile eveyr now and then in winter is good.
That is what I can think of for now...be safe and have fun
.
(I personally like to use a fuel additive, as do some others here.. Millers Diesel Eco power max works nicely in my Bongo, you can look into this - they have a petrol version as well, it helps keep engine insides clean/injectors/fuel lines/lubricates some areas etc. I also once in a while like to use a BG product either BG44k or BG244, but this can be every 2-3 years or so. This bracketted section is purely optional and the above is most important.)