Id imagine an ordinary house drill wont drill through stainless steel. Or does it?
As long as the drill is either a 3/8" or 1/2" chuck it should do fine... I used my DeWalt 18v cordless to go through two 4mm thick aluminum pots to convert them into kettles. A corded drill should have plenty of power to go through SS, provided you get the right bit for the job...
It really is critical to get the right bit, or hole saw, to cut through the metal. You might get one (or two) pots done with a bi-metal hole saw, but you'll get many, many more with a good grade step bit (titanium coated, or cobalt).
Look at it this way, for the cost difference... If you already have a decent drill, that's great. But, you'll need a good bit for going through the SS pot.. That can be ~$10 for a bi-matal hole saw, or up to almost $50 for the step bit. Then you'll need to spend about $30-$40 for the valve assembly (depending on where you get it, maybe more, maybe less, but think about shipping costs too)... So, you could be spending $50-$100 to install that first ball valve. Granted, if you get the good bit, you can space that cost over how many times you use it... You'll also want to get some cutting oil to keep the bit lubricated while using it, so you don't burn it out. I've not priced that out lately...
If you know someone that has the tools to make the hole, then that helps. Otherwise, it might be cheaper (and give you more peace of mind) to get it with the valve already installed.
Something else to think about... You'll want some decent files, and/or emery cloth to smooth the edges of the hole you make. Otherwise, you'll cut the o-rings that come with the valve assembly... I did that on both pots I converted. Luckily the LHBS has replacement o-rings (thicker too, so they actually seal better)...
I will say that once you have the ball valve in a kettle, you can't imagine not having one. It makes pouring into the fermenter so much easier. I'm using an IC to cool my wort, but with the ball valve I have the option of other chillers later. If/when I get a SS kettle, it will be fitted with a valve either from where I get it, or before I use it the first time (most likely, from where I get it)...
BTW, you need a 7/8" hole for the 1/2" ball valves... In order to step up to that, you'll need a drill with a 1/2" chuck. Unless you use a step-bit...