I have the exact same Mash Tun, not sure of the brand but it has Daddy Cool on the lid, just like GMK's. I found it in my parents garage when I was scrounging AG gear and have never bothered upgrading. Where the tap is located it's got a flat section, so installing a bulkhead is no problems, and it seals perfectly.
It holds the temperature really well, but works best when it's 80%-100% full. Not sure how many liters it is, never bothered measuring. I've had 5kg grain with 12.5 liters of water in this tun before, no worries. That's enough grain to make 21.5 liters @ 1.050, assuming 70% efficiency (although you will lose some of the final volume to dead space in your kettle). If you're like me and make lower gravity beers such as Ordinary Bitters you can easily squeeze out 28 liters @ 1.038. When you're starting out this size mash tun is all you need, and for someone who likes to brew small but brew often, it's all you will ever need. It's only when you start making high gravity beers or double batches that you will need something bigger.
Wee Stu, you'll love this wee mash tun. The Daddy Cool is tops!
Cheers
Mark