Hi all - can't believe how long it is since I've been on here! Hope everyone is well

We've an appointment at the vets this teatime but I know there's folks on here with a lot of experience so here goes......
My remaining cat Buffy (FIV, about 13 yrs old) has luckily never had any medical issues, but before Christmas she went off her food and was being sick. Her temp was fine and full bloods showed no problems, but this still took a couple of weeks to resolve.
On Friday morning early hours, she was a little bit sick, then she hid all day and refused to eat although she was purry when I went to check on her (regularly!!). There was a bit of diarrhea in the trays. On Saturday she had perked up a lot and was licking the jelly off the food - she must have heard the vets being threatened! Sunday she ate a little less but was ok in herself. In the litter tray, there was a piece of poo that was pretty much mostly hair. I was out overnight and when I've come home today she's not touched the food the cat/dogsitter put down for her, and she was so hidden it's taken me ages to find her.
She's had a little purr and then done some of the furball type coughing and curled herself up away from me again.
Definitely time for the vets. No doubt however bad she's feeling she'll still put up a fight about getting in the box!
I've had a few cats with long hair and never had a problem with furballs. It might be that this isn't furballs and hopefully the vet will be able to sort whatever it is but I was wondering what experience folks on here have had with furballs, does it tend to affect older cats? I remember someone mentioning a paste they put on their cats leg to help with furballs?
Thanks
Su