The cargo is a good harnessed seat - just make sure it's one with a front adjuster. That model number brings up the "platinum" in google, so I think you are OK.
Given that your child is 35 lbs, it won't last as long as the Nautilus, but for free that is fine. If you were buying it, it would be better to put the $80 toward a Nautilus, IMO. It will only have about 20 months before it expires, so keep that in mind, but again, for free, that's OK and he will likely be close to 40 lbs by then. I think it would be a great stop-gap measure for your DS and then you can find the next greatest seat when that expires.
Mama to three boys ('03, '05, '07)