I have a trade account due to the amount of batteries I had to buy for all the rusty toys my dad left behind, when I get a battery I go into the warehouse and choose it, they then take it to a bench to do some tests and then label it up.
When they tested my t5 battery they knew I had just charged it by testing it but under load it failed the test, a multi meter cannot do this test as I think it needs to be under load, you could try switching everything on (lights, stereo, heaters, heated windows) without the engine running and see how much the voltage drops.
I think with the engine running it should be about 14.5v but you can search for your car details on the net.
I don't think there is any point in getting a start stop battery as your alternator is made for the correct battery for your car.
When I was on the tools my van used to do about 1-2 miles between jobs about 10 per day and I had loads of equipment running off it, it always started after every day for three years before the van was replaced