Don't cater, not on a budget. Have family help cook. 2 of my mom's friends cooked for my wedding, about the same size as yours. We had salad, ham, fruit and veggie platters and a potato dish. Really simple. My cousin just got married and they had an ice cream bar, they just rented a soft serve machine, so I'm guessing pretty affordable, and I'm not sure what you would call it but about 8 different chip/cracker/veggie dips, both of which I think are great ideas.
here are some ideas that may be more cost effective than real flowers,just check google images for wedding centerpieces. They have lots of tealight floaters, and various size vases with candles you can fill them with rock or with sand in your wedding colors.
another neat idea, instead of doing favors for the guest my cousin had a little sign in a frame, typed all pretty, explaining that they made a donation tot he local animal shelter in their guests honor instead. The frame was next to the guest book so everyone saw it, and no one ever needs to know if the donation was 100 bucks or 20.
as for invites, you can find do it yourself kits at any craft store and even places like walmart. it's a computer program, you enter the info and print them on your printer at home. Cheap and actually really easy to do, and I don't see anything wrong with rsvp'ing to an email. Maybe you can set up a free wedding page online where people can rsvp?
If it's a small wedding forget a program, chances are most everyone knows who is in the wedding and all that stuff.
Also, if you can avoid buying decorations, then avoid buying them. I wish i had thought to borrow more, I have about 25 vases I will never use for anything. Freecycle can be a great resource for finding things people are giving away rather than throwing out. But if you do need to buy things I was really happy with the stuff i bought from Oriental Trading company, and they are cheap.