Needles, I have a light weight Brother that I carry around. As a matter of fact ,I'm at my DD now & have it. Already today a friend of hers was eyeing it. I just looked all over it for the model but don't see it. I think its a 700XR or similar. I got it on Overstock. Its reconditioned, but computerized with 70 stitches. It only weighs about 7 lbs or less. I've had it about 5 years and never had a problem with it.
as to the seam allowance, yes it can be different with each machine, etc. I found a neat gadget at LQS called Perfect Piecing Seam Guide--don't leave home without it! Its from
www.perkinsdrygoods.com
i use more than one machine on most of my projects & this little yellow plastic thing has made a world of difference
good luck on finding your machine