Our stories are so similar! I bought my 18 yr old Friesian mare in December. She had also been out of work more than a year and a half and kept shod for years. But when I went to see her, I instantly loved her.
She was fat and out of shape, so as soon as I got her, I started her on a quality senior feed (Triple Crown) and then shortly after, I started her on the Smartpak Senior supplement. After bringing her back into work (riding 5 days week) she quickly lost a lot of weight and did not have much muscle. In February, I put her on Smartpak's Muscle Mass supplement and had the chiropractor out to do some much needed adjustments.
When I got her, she couldn't hold a canter for more than a few strides and panting heavily by the end of a 45 min lesson - mostly walk/trot. Now, only 90 days later, she has no problem with canter work and gets through our lessons easy peasy.
I, too, was nervous about considering an 18 yr old horse for purchase, but the current owner and a previous owner I found on FB said she'd never been sick or lame a day in her life. In addition, she passed the PPE with flying colors. If the horse is in good condition, and can pass PPE, I wouldn't discount him. We have several horses at my barn mid to late 20's still being ridden regularly!
Below are pics from the day I got her - she's so fat, you can see the dimples in her butt! Lol. And a couple from this weekend and last weekend. She doesn't even look like an 18 yr old horse anymore, and we plan to compete in our first dressage show next month!