My MIL gave us an engagement party as they live in another country and many of their relatives would not be able to attend the wedding. Every one except maybe one did give a gift (which surprised me, I didn't expect any).
Also for the wedding most every one gave my inlaws a gift to give to us, even if they had given us an engagement party gift, but they were 'smaller' gifts, which was still more than I even expected.
I think it is in bad form to mention gifts any where but on the shower invitation, but this is just me. It is so common now, that it is taken for being correct, but really, it's not.
If you do want to bring a gift to the engagement party, fine, if not that is fine too. Again, I was surprised most people did from dh's side of the family. I'd make it small, just a token gesture. You are under no obligation, whatsoever, in a wedding, to give any gift unless you are invited to the shower - then you ARE obligated because that is what a shower is for - to shower them with gifts for the new house. But that being said, I've never heard of going to a wedding and not bringing or sending Some kind of gift. So if you want to for the engagement party, do so. If you can't, or don't want to you are under no obligation whatsoever to do so.