Man o man! Old Poi, you are going to be in for a treat once you get your offset no matter what brand you buy. You have never truly experienced the "experience" of barbecuing and smoking until you get one.
Do you have a price range in mind? I think that is going to be the determinant on whether you have to modify or not.
Im guessing now, that any offset you buy for under $500 is going to need modifications. Once you get to the $500 to $1000 range (and these are new, retail prices, by the way) the unit should be good to go as-is.
My opinion is the Brinkmann Smoke N' Pit, for under five-hundred. Two reasons....I own one and think it's the best for the money, and two: It's counterpart- the Char-Broil or Char-Grill has a negative history with fighting to put the burner box on. A recent post included something about a wife threatening to leave....
due to the anger, anxiety and angst trying to put the darn thing together.
(Sorry~ had to put that one in. I still think that was one of the all-time "funniest-threads". If I recall, it included a hammer, clamps, a couple bricks, 2x4's, a sledgehammer, a drill, a chisel, a sawsall, several trips back to the point of purchase, and according to the beautiful Mrs. Rivet, "probably several sticks of dynamite".
A quote from the above experience:...I went immediately to Home Depot to get a 2x4 after work. Got home and put the chamber on its side. I wedged the 2x4 into the body of the main chamber and whacked away with a hammer. And I killed my smoker. The football is still in there good. I bent the heck out of the whole right side of the chamber. I hate you Char-Griller and all you stand for. Okay, that last bit was a bit much, but I am really frustrated.
I'm going back to Lowe's to demand a refund and that they remove the football on my replacement. The one I got is toast.
It shouldn't be this hard.
My recommendation as a completely biased offset smoker-guy: The Brinkmann Smoke N' Pit.