thinking of replacing a\c and furnace seeking knowledge from the hivemind

TLDR; old house (40s) load calc suggests 2.5 ton, considering 3 ton to help with cape cod upstairs, not sure if that falls into the "don't oversize" or .5 ton is ok. Also is it worth getting a heat pump in an older home?

Longer version:

So I have 25+ year old a\c and furnace. I live in southern ohio in a home built in the 40s (no wall insulation, some in the attic, cape cod style brick construction, 1380 sq ft)

Current unit is 2.5 tons and on warmer (90+) days it struggles to keep the downstairs at like 75-76. Up stairs sits a 79-80, but there is only one supply per room up there so I'm not sure that will be fixed with a new unit.

So anyway, my question(s). Had a load calc done, and it recommended a 2.5 ton unit for my space. The contractor after hearing about my cooling problems upstairs and looking at my ductwork said he wouldn't be opposed going up .5 a ton to a 3 ton unit for my space. I've read about the problems oversizing can cause, but the way he explained it is that .5 a ton isn't a huge jump and may give the upstairs just a little more push with the extra cfm to help the temperatures up there a little. Does that sound right? I definitely don't want to oversize, but it's not like i'm going up another 1-1.5 tons over, it's .5, do you think that will cause problems or I should stick with the load calc of 2.5? I do plan on adding addtional insulation in the attic, but it is a cape cod, so I'm not sure how much that will help.

Also, with an older home does it make sense to spend more and get a heat pump, or does that make more sense with newer construction?

Currently I've been quoted for an 80k btu Amana AMEC 96% 2 stage, 3 ton ASX16, new humidifier, new lineset, and reworked plenum for ~8.5k which includes tax and permits. Then I have rebates\tax credits of 700. Going into it I figured i'd be around 7kish so with the other stuff I'm having done (humidifier, plenum, tax and permits) it sounds pretty reasonable to me. I have a few more quotes coming in but wanted to get some input from you all as well.

Thanks for your help and all you do!

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