I've been to a couple of engagement parties recently and both of them just hired a pub function room with a minimum spend and expected the guests too buy their own drinks to make up the number.
I'm not necessarily saying it needs to be an unlimited tab behind the bar, but if I was throwing a party for my own engagement I would at least put on a few platters and provide a welcome drink. If I couldn't afford to do that then I wouldn't bother having a party in the first place!