I'm left-handed too (and play left handed), and was told the same thing. I have a feeling all lefties are told that when they first buy a guitar
Its not entirely bad advice. Its a heck of alot easier and probably cheaper getting a good right handed guitar than a good left handed guitar. Its soooooooo frustrating when I go to a music store and not be able to play 999.9% of the guitars, and IF they do happen to have a lefty its always a lower end model. I decided to get a lefty guitar though because I was afraid I'd just never be able to pick with my right hand. It just felt way to weird. However ALOT of lefties go the right handed route, so its not necessarily a mistake if you go that route. I mean it can be done and is done alot. Its really your call. I chose to play left, while I don't actually regret it, there are days I kinda wish I could play righty cause I'd have a lot more choices of guitars. But then again nobody ever asks to borrow my guitar.
As too what you should do or shouldn't do, I'm not entirely sure what exactly you mean by that, but I will say IMO I don't think you should start out by buying a super expensive guitar with all the trimmings. But you shouldn't buy a 50$ piece of junk either. IMO a nice middle of road starter guitar. maybe a Mexican strat or something in the price range of 400 bucks or so. I'm not recommending anything just try out as many guitars you can and pick the one that sounds the best and is most comfortable for you. In time, after a few years or whatever, you can always upgrade to something better if you want.
Just my opinion