A car crash is an extremely violent event. It needs to be able to hold onto several hundreds pounds of force at a given moment. Imagine a couple of 200 lb men hanging onto the harness straps. Brittle plastic during a crash could potentially crack near the harness adjuster strap. The harness could rip through the shell and eject the baby. This is a hypothetical situation, but it could happen.
A manufacturer also puts an expiration date because parts gets missing over time. Do you still have the manual? If you do not have the manual, can it be found online? Some car seats are incompatible with vehicles. Did you buy a new vehicle? Can this seat be correctly installed in the new vehicle?
I have not seen the car seat and I do not know how it was used or stored, so I do not know if I would use it in your situation. It depends.
Children should ride rear facing until they are at least 2 years old. Here is why: http://www.youtube.com/watch?v=Q8gU9zzCGA8
Saving money is extremely important in this economy. I can totally understand. Safety does not need to be sacrificed. In fact, getting a convertible car seat will be the best financial decision, better than getting a combination seat (forward facing harness to booster). Take a look:
If you want an infant car seat get the Cosco Comfy Carry Elite Plus ($70). After your baby outgrows that, buy the Safety 1st Guide 65 ($70) This is a convertible car seat that goes from rear to forward facing with the harness system. This seat should last you to age 3 rear facing (if you choose to rear face this long) and until age 5 forward facing. After your child outgrows that, get an inexpensive high back booster seat ($50).
A seat that goes from forward facing harnessed mode to a belt positioning booster usually do not make the best boosters as they do not allow growing room. In fact, one model, the Evenflo Securekid only gives you about an 1 inch worth of torso height room growth after outgrowing the harness by height. I find that booster seats whose only job is a booster, do a better job of positioning the seat belt. It is also much easier to move between vehicles.