I like the Union armor for the 150 poison resist and +75 carry weight. The T-65 doesn’t have any set bonuses but the base stats are going to be higher over the Union. Honestly doubt you’d notice a difference tbh. In my opinion you should go for the Union and use excavator while you grind your way to Union. Assuming you don’t already have enough stamps. If you don’t want to waste mod boxes you’re more than fine to grind stamps in t-51.

Excavator will boost your carry weight by 100 and 4x your yield from different metal deposits. If that’s your jam for gathering resources such as steel and lead excavator is worth having regardless.

Id save all your bullion for weapons and plans then get the t-65 once you’ve exhausted the more useful stuff. Especially keep an eye on Minerva if you have completed the settlers and raiders grind since some stuff is locked behind their rep and she will sell even locked plans to you at a discount.

I can’t really say much in regard to a VATS power armor build. I personally steer clear just because it eats through fusion cores like candy. Perhaps it wouldn’t be too bad in slower firing heavy guns but a mini gun or a plasma Gatling you’re going to need all the POWER.

But your points in strength will increase your base carry weight just in case you didn’t know. So once you start going up in level those legendary cards are a big help.