Hi there and welcome to TPU
As
@ThrashZone says, the 3080's have 10GB of VRAM rather than 12GB (although I believe there's
rumours of a 3080 Ti with 12GB?? How true don't know, but meh, we've sorted that out
)
As for the pay back side of things... Well, currently prices for the GPUs are if you use Ebay as an example... Crap. I think from a post I saw earlier, a 3070 is about £1200 and a 3080 £2000 over in the UK (where I'm based... Whereabouts are you??) so with your 5k, I'd guess you might be lucky and possibly get 2 cards with some change left over, maybe enough for the rest of the system (if you've not bought it/have it already) so that side of it, it's going to go very far.
So with regards the cards a bit more depth to them, they do require pads to be changed in certain models. From Reference/Founders whatever you want to call them, they will definitely need doing. Think about $15 should sort out a card but then you have to take them apart, risking the card etc.. Bit of a pain.. However I believe the RTX 3070's don't have the memory issue with the RAM, so, you shouldn't need to replace the pads at all on those cards.
How it's seemed to be working so far..... Profit stays about the same roughly, a bit up and a bit down, depending on the block reward and the network hash rate. So go to a calculator for Eth as you wish to mine that, put in your estimated hash rate for the card/s you wish to use/hope to get, make sure you check your electric costs as well and see what the profit is like. A single 3080 will net you a fairly good return per month, but because of the difficulty you will need to increase your hash rate continuely to get a decent return because as an example, 30 days ago the difficultly was 25% less than it is now. 90 days ago the difficulty was a massive 70% less. (
My source - ETH Difficulty) So if the coin continues to get higher in value, you can only expect the difficulty to rise.
I'd really look into costs for electric, where you can get GPUs from, if at all and make a bit more of a decision based on that information. Paying too much for a card will just take too long for it to be paid back and if everything went south, you'd have lost money from over paying too much for each card. If you have something you can use now without buying new hardware, give it a go, check the power consumption and all that, it'll be your biggest outlay (bar buying the cards)
I really do hope that's of some help.....