#1
Okay, so my plan would be to buy the Fender Thinline (prettymuch a semi-hollowbody telecaster) with the two Single Coil pickups, put another single coil in the center, and 5 way switching, to make it strat-like tones, with a semi-hollow body. Would there be any problems with this, and how much would it run me?
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#2
you'll have trouble getting true strat tones without a full set of strat pickups, but barring that if you put a reverse wound reverse polarity strat pickup for the middle you can at least have that middle pickup strat sound as well as having positions 2 and 4 be hum cancelling. the thing that is a bummer with that setup is that you lose the tele combo of neck + bridge, but if you think you'd like it better the other way than go for it.

price depends on the quality of the pickup you choose. you could go gfs and get one for $25 if you're on a budget, or you could go mid-range and get a fender noiseless, or even better get a lace sensor. your choice.