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As the other poster noted a 1/7 twist was made for the heavier stuff then 55grn.  It will stabilize anything up to 80+ grains.  Whoever said that probably meant it the opposite way, saying anything heavier then 55 grain.  As anything lighter then 55 like 45-50 grn would probably still be OK, it depends on each persons barrel.  

Honestly the best ammo for the price is Wolf Gold IMO if it is range ammo.  You will get a lot better accuracy out of it then the steel cased Wolf, and generally it is only another $50 or so a case.  Plus in a worst case scenario you can use it for SHTF or defense etc and not be too far behind a full powered M193 55 grain round.  The Wolf Gold is loaded a touch lighter then full powered, but very close.  The Wolf steel cases, and many other 223 rounds like PMC are loaded even lighter and I would avoid them if possible.  If you are on a very tight budget then the steel cased Wolf/Tula may be OK, but I would not want to use it for anything other then range ammo, and even then it is going to have a lot worse accuracy compared to the others.  

Most barrels are chambered in 5.56 instead of 223.  But if your chamber is a 223 then I would stick with something like Wolf Gold and not get anything hotter.  But since it is more then likely a 5.56 you can shoot any 223/5.56 you can chamber in it.  Let us know what type of barrel it is and the markings on it and we can help you with that.  

For bulk stuff here is a good breakdown from my knowledge on the subject:

For cheapness:
Wolf/Tula steel case - OK - bad accuracy very dirty and not full powered so may not function in your rifle very well, or be good for defense.  Can be had for $230-240? a case plus shipping.

For accuracy:
Wolf Gold  - it is loaded almost to full 5.56 specs but should work well in a 223 chamber as well.  It is said to be one of the most accurate bulk ammos and seems very clean shooting.  Would not be bad in a SHTF/Defense situation although there is better out there.  It can be had for around $280-290ish a case, sometimes $300 shipped.  Don't forget the brass casings are re-loadable and that almost pays for itself over the cheaper steel case IMO as you can always sell the once fired brass to someone who reloads if you don't.  Then have about the same in a case as the steel cased which can't be reloaded.  

For full powered M193:
IMI M193 - Best all around ammo in my opinion.  It shoots about as good as the Wolf Gold, if not better in some peoples rifles.  It is loaded to 5.56 specs so is full velocity, brass cased and very clean shooting.  Out of all the full powered m193 rounds it is probably the most consistent.  Price is usually a good bit higher then Wolf Gold though, and for just plinking at the range it is probably not worth the difference.  But if you plan on shooting it at anything with a heartbeat then this may be the best bulk ammo you can get.  Usually it is around $330-350 depending.  

Federal M193 - A little less accurate then IMI usually, but still very good ammo if it is all you can find or you get a good deal on a case.  It is full powered as well, and who knows maybe your rifle would like it better then the others.  You  never know until you try a couple boxes of each out in every rifle to see what goes with it.  Price is about the same as the IMI at around $330-350

Also I have heard really good things about CBC/Magtech M193 - I think it is loaded to full power as well, and I hear it is very clean and accurate.  I would probably put it in about the same category as Federal if not better, but have never actually shot any of it myself.  

All of this is just my opinion, but hope it can be of some help.

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Anything cheap that goes bang for range ammo.  I would not rely on range ammo for self defense unless you have no other alternatives.

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I have never shot any PPU M193 myself but would recommend it myself as well from the other PPU I have shot.  I think it is loaded to full power and from the other calibers I have shot the PPU is pretty good stuff.