It depends on the OLD of your bike (over locknut dimension).

Most 27's are 120 rear, 100 front and I would not think yours is an exception unless you actually DO have a "cassette". (Older bikes like these used 5/6 speed freewheels - not wide "cassettes")

Niagara cycle has a selection of inexpensive 27 rims and wheels. The Sun rims are well regarded though I've no personal experience with them.

You can use 700c's on bikes intended for 27" but very often the 4mm smaller wheel radius forces you to find some alternative brakes with a longer reach.
Some people want to do this because they prefer some 700c tires, but others avoid it, since there are excellent and plentiful 27" tires available for comparatively less money.