We have a Britax--can't remember the model name but it's convertible, easy to install and use, and has a high weight limit. It's rated very highly in crash tests and by parents in terms of ease of use.
If you hit Babies R US during a sale and take coupons, you can get this seat for half the list price.
Whatever you get, buy two bases, not two seats, to save cash.
The easiest and safest way to handle installation is to make an appointment with the safety officer at your local police station.
Lastly, for equipment decisions like these, check out the book Baby Bargains. They rate various options, discuss parent satisfaction, etc. Great resource! You can buy a copy for about $20 or check it out from the public library for free.
Best of luck.