I agree with the comments about why host if you can't afford an engagement party!
Like its not an essential to have an in fact its kinda a yay look at moi. (I don't mean this in a bad way as I too am looking at having one). I just think if you want the extra added attention of an engagement party then you should front up the costs and if your lovely friends and family want to donate something, then wonderful...I certainly am not implying on my invites.
We are going home to NZ when baby is 3/4 mths and because I never had a baby shower and I know I have a lot of friends and family who want to meet the little one, we also got engaged last year so we can kinda tag it into one thing. My mother has been already asking whats happening. I'm thinking a weekend arvo get together BBQ - more a social gathering and we will supply meats/some salads and basic drinks/beers. If others want something more they can BYO.