I'm a SAHM to 2 year old twin girls so congrats on your twins!
My advice is to get the type that tie onto your chairs. I like the Fisher Price Healthy Care ones the best b/c they are super easy to clean and have a tray lid and tray you can put food on in the fridge and just pop it onto the tray. My girls have been using these since about 6 months. I also have a set of the First Years at grandma's house that are good, but the tray swings to the side and I just don't like them as much. They are also easy to fold down and travel with if you go somewhere.
If you get these kind then if you have a kitchen island or a big countertop you can put the non-stick shelf liner paper down and then put the chairs on top. This is great when they are little and saves your back and a mess on the floor. BE CAREFUL and don't leave them alone and stop when they can be big enough to tip over. Duh. :) A lot of times now if we go somewhere to visit I just put them on the floor with the back against the wall and the girls eat in them without being tied to the chair.
If you want to buy a used highchair I think it would be fine as long as they seem clean and you clean them first.
Good luck!