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6' 1" sounds waaay tall for a 54cm/21" frame bicycle; this size would normally be for someone around 5' 7" - 5' 8"

i am 5' 9 3/8" and 56cm is my size

would guess your size to be in the range of 61-62cm for a road application

you asked about rust removal -

for frame your best procedure would be to immerse in an oxalic acid bath; this takes away all of the rust without harming paint or transfers; you may be surprised at how much of the paint colour will reappear. it is available in bulk as a white powder one mixes with water. lots of information regarding its use is available here on the forum.

no guesses as to parts costs to put the cycle roadworthy. there is the obvious material such as saddle, cables, cable casing, brake pads, handlebar wrap, tyres, tubes, bearings, possible chain and gear block...then there are items which may be a surprise such as the wheels...

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