In terms of safety features, Side Impact Airbags were optional in '95 and then became standard sometime after that. TRACS traction control was replaced by the more comprehensive STC in '99. The IC (inflatable curtain) airbags came out with the redesign for '01. All of the other safety features are pretty much standard.

In terms of newer models, the '98-'00 V70 has a better ride and more updated interior from the 850. The '01 redesign is a totally different car based on the S80 P2 platform. The best deal IMHO is a 3-4 year old off lease model. They tend to have lower miles and per the lease contract had to be maintained so records may be more available. Volvo (like many other mfrs) leases a lot of its cars so the prices tend to plunge when the market is flooded with off-lease units.

Sportwagon is just a generic term and I have seen the badges on '94 wagons, the first year in the US for this model. The T5 has the high pressure turbo motor and the R I think has a larger turbo and more aggressive ECU mapping for higher output. Starting in '97 the GLT has the low pressure turbo, which is a good compromise for many.

Childseat is optional. The 850 wagons and '98-'00 V70s should have identical interior dimensions. The '01 V70 has a little larger passenger volume but a little smaller cargo volume.

I personally would be a little wary of buying a car on ebay because I think the pictures sometimes don't tell the whole story and I would want to have any potential purchase checked out by my mechanic first, but that's just me. Good luck in your search.