A 99K is a 3/4 size machine compared to other models like 66, 15 and 201. It's smaller and weigh less, but the Featherweight is an even tinier cute little thing, and weighs much less. I dare say your 99 is a better machine for use at home mostly because of the larger harp and bed space, and the drop-in bobbin case is nice too. It's sturdier and you don't have to be afraid to over load it.
I guess a 99K would have been converted to 110 Volt electrics in the US, the US made 99s doesn't have the K, but are exactly the same machine. The sub letter K was given to machines made at the Kilbowie factory in Scotland. There are a few variants
to model 99, and it was once a very popular model for it's ease of use and dependability. The old motors can get hot, especially in summer. What helps is a cleaning and lubrication of the motor, and not to over load it; take a small break between each bobbin you need to fill if it get's too hot. The speed controller can get hot too, and again, what helps is service and adjustment. They give off heat, that's the way they work (slowing the electric current), but should be within reason, slighly warm not hot.