You've basically re-invented sous-vide cookery for steak. Steak is about the easiest food to cook sous-vide. You can find a lot of resources describing it on the web, but here's a few notes
1) 170F for 2 hours undoubtedly left your steak very well-done. Not a problem if that's how you like it, but also not necessary. You can get medium rare by cooking in the 130-135F range for two hours. Cooking sous-vide below 130F (necessary to get steak rare) is not recommended for safety reasons, as temperatures that low can encourage the growth of dangerous bacteria.
2) With sous-vide, there's no need to rest a steak after searing it. Resting grilled steaks is necessary to give heat time to move from the outside of the steak to the center, and actually get it cooked to the desired temperature. With sous-vide, you already did that in the water.
3) With an impromptu sous-vide setup like yours, you probably don't want to cook for more than a few hours. You risk your temperature getting too low overnight, and then you're down in the bacteriologically dangerous ranges. You can get some great effects cooking longer (I had 24 hour marinated skirt steak last night that was amazing), but you should probably do so with powered equipment. There are home sous-vide machines available (a bit pricy) or you can build your own. There are dozens of different rigs described on the web. I built mine for about $100.
4)Longer cooking times are better for tougher cuts anyway. If you're just looking to do ribeyes and strips, then two or three hours is fine.
5)You can marinade, but obviously not in supermarket bags (unless the supermarket marinades, e.g. pork tenderloin). To do so, you either need to purchase a vacuum sealer (not too expensive), or be clever with drawing air out of ziploc bag and sealing it very tightly.
Attempting to cook a steak in that pot may cause it warp—its nowhere near as tough as a cast-iron pan (which, by the way, are under $20; the Lodge ones Walmart sells are fine). Your pot probably also doesn't have the heat capacity required to completely sear a steak (how heavy is it? If its not at least several pounds, it doesn't). If your pot has a non-stick coating, you probably don't want to do this—there is a risk of overheating it. (The cast iron won't care if you heat it to 600°F, but nonstick coating will)
If you have access to a grill, that's a much better bet.
If you have an oven with a broiler, that's a good bet, too. Get the broiler hot, put the steak very close to it, and keep an eye on it. Flip when browned. You may have to finish in the oven if you don't want rare or medium rare.
If you want to cook beef in a pot, I suggest a pot roast or stew. But those don't use sirloin.
Best Answer
water, seasonings, aromatics vegetables (carrots, celery, onions garlic... ), tomato paste and/or canned tomatoes
Brown the meat in oil in a dutch oven type pan, remove and set aside.
Brown the vegetables in oil, when good you have coloring (not burning), add tomato paste, mix and cook well for a few minutes.
Add meat back to the pot, add water and canned tomatoes to nearly cover the meat.
Reduce heat, cover with a lid and cook until meat is tender, adjust water level so it does not dry out during cooking
You can do it on the cooktop or in the oven.