Trying to piece together a bike is quite frustrating and expensive. It is better to be patient and buy a couple or few bikes on craigslist that have some (or all)of what you are looking for.. Cruiser componentry hasn't changed at all in the past 60 years, (stem eand seatpost sizes may vary) Derailleur cruisers are almost all 7 speed, and have been since about 1995.
The Electra Townie or Cruiser Lux 7d are good aluminum bikes. The Lux 7d is not as common as the steel 7d. But they still aren't heavy like a 1953 Schwinn Hornet
electra uses stainless steel hardware (an added plus)