I need some advice from seasoned party-throwers for a 21st...please??!!

[Haylstones]

The only parties I ever do are kids' ones so some advice would be good.
It's my brother's 21st and I've offered to organise a surprise party and I need tips and advice on all areas . Think we'll have about 50 people, maybe more.
We have a DJ and a hall booked, I know what cake I'm making and that's it
I need some tips on:
food
drink- is it ok to do boyb? And should I buy some booze/mixers.soft drinks/tea/coffee?
decorations
home made invitations (need to be printable to be ready in few days)- it's a surprise so wording/rhyme advice would be good
any other nice touches/ momentoes.
I'm also not in a position to be frivolous so budgeting tips would be much appreciated

BUMP

Cheers, have managed to do OK invitations now so just need to print them and that's one thing ticked off.
It's in a village hall and no bar so was thinking of providing a small selection of drinks and invite people to bring their own. Lots of ppl have parties here and it's the 'norm' to byob but wasn't sure if it was always ok. No way can I afford to pay for drinks for all theses people!
Any ideas on where and how to decrate the slightly shabby hall?

Thanks Ruby, am going to hire some helium and do some balloons. I've got a few decorations but need to get some more.
Where is the best place to buy buffet style food and what do I buy? Ages will range from toddler to 70s - expect there will be at least 50.
Advice much appreciated, really want the party to be fab for my fab brother!