could you put mid range, tweeters, and subwoofers in a cab and get a sound like a normal full range speaker
Not if you are playing a guitar through it. Normal speakers are not rugged enough for guitar. Typically a tweeter, midrange and subwoofer is the best combo, but with guitars it's not. Not only will it not be rugged enough but it will also have too much high end and too much low end. Part of what we look for in guitar speakers is just the right amout of limitation in frequency responce. If we didn't want the limitaion then we would all play through a PA.
the stores around me dont sell speakers like guitar ones and cab are pretty expensive and i would like to design my own
well, if you want a half stack its cheaper to buy, but building doesn't work out much cheaper for cabs, maybe a 2x12 you'll see benifits, but generally the price isn't a lot lower than buying a cab, at least not in the UK