My thoughts, It will all depend on how much money you have to spend. The Juki 555 (early 80's) is probably alot newer than the consew 18. (early 60's machine). About 3/8" of lift as sews 5/16". You could change the needle system to a 135 X 16 and sew a little more but you would have to take out the upper tension release pin as well to sew maximum lift of the machine. Probably pushing the max with either one.
Like medic says, a walking foot is better, but they cost more too unless you can steal one somewhere. if toledo or some of the other sewing machines guys had a 400.00 walking foot like a consew 206, 226 or Juki LU-563, you would have probably got an email from one of them by now.
Thing to remember - check the RPMs of the motor when you look at used machines. You dont want a motor rated at 3450 RPM's if you can help it. It will run fast.
DC Servos are the only way to go and if you can, put a speedreducer on it for that extra slow speed. That will cost you another 2 to 3 hundred bucks but is worth every penny.
Goood Stitching
stitcher1