Like any form of entertaining for a large group, if you want to do a BBQ to this level, you need to consider whether you have the facilities to manage it and if you don't, then yes, get someone in.
We host/co-host/attend events like this fairly frequently (like a PP _ South African here and it's fairly common)
First thing though - unless you're planning wedding-style dancing and partying until dawn with the requirement of a second meal, I would only plan for one meal, but a substantial one. Lots of snacks/nibbles. Then the main food. Then plenty of desserts. Then leave leftovers out wth bread etc for people to help themselves from as they like. Whether you're catering the main yourself or getting it brought in, think pates, charcuterie, crisps and dips, olives etc to have out and ready for people on arrival.
Then in terms of self catered or catered - it depends on what you can manage. If catering, lamb or pork ont he spit is a great choice, you can normally get good sides catered as well, they do a lot of the tidying up and it's all very easy and painless.
If doing it yourself, you need a decent sized BBQ. This is not goign to work if you've just got a small coal BBQ. You need a big one and one that you (or whoever is in charge of this part of things) can use effectively. Gas, obviously, is a lot easier when cooking for a crowd.
We quite often do a huge pile of sausages to start for the children (and anyone who is ravenous), then cook the rest of the food (which the children can also eat) on an ongoing basis. Chicken thighs/drumsticks are great in that you can do a huge batch slow cooked in the oven then finish quickly on the BBQ. Similarly ribs or chicken wings. Lamb chops, marinated, are fab - choose thinner ones that cook faster.
I love things like kebabs etc but quite frankly, preparing for 30 is a pain so a compromise is to get your local supermarket or butcher to do that for you.
Salads and sides when self catering - consider what space you hvae to store them. I'm inclined to 2-3 large salads - a green salad, a potato or couscous salad and a tomato salad usually. And lots of bread. I often buy par-baked ciabattas/baguettes because you can do them in the oven, they're fresh and if you need more you slap more in the oven.
Again, thing about the logistics of tidying up. 30 people means a lot of plates etc. Do you have space for that? Do you need to use paper plates to manage it better?