Rear disc brake conversion process question

[full foot notch]

So bought a truck now nicknamed the mystery machine as im the third owner. First owner built it. Second owner didn't know much. In the ad he said it had a 700r4 posi which I think he mean to convey was that the rear was posi and not the trans. I believe he also mentioned something about the rear being a 373 gear ? Anyways I've got lined up to use impala SS rotors and calipers along with captain fab brackets from off here.my local mechanic quoted me $500 to do the job with me supplying the parts. And since I've gotta make every dollar stretch I was contimplating doing the job myself. From what I've seen it's draining and removing the rear cover. A bolt for the cross pin , push axles in and retrieve the c clips. Then pull the axles. Unbolt the line and backing plates and reverse install the other stuff. My questions is how can I tell if the inner and outer axle seals need to be replaced and how hard is to do with no air tools and basic mechanical knowledge ( I've done water pumps and alternators and starters ). I'm assuming the second owner is correct in that it has a posi but how can I inspect to make sure it is what he says it is and the gear ratio he says. Also any things I need to look over while it's open.

Also for brake lines I was planning on going stainless braided on front, braided rear center link and thinking do I use factory rear lines or order pre molded replacements and shorten them and reflare the ends for the impala braided hoses.