Like I said, find a real steel yard. You can get a 20' length for what a box store like TSC or HD will charge you for a 4' piece. Plus the steel will be of a better quality and not all bent up. If you plan your joints right, you can virtually eliminate having to cap tubing. You can cut steel to weld caps on, or there are plastic press in caps for the common sizes of tubing. An industrial hardware store will have them, or you can get them online. Also many steel suppliers will have them. You can also get ones like
THIS that have a caster socket built in, so you can pop in a stem type caster, no muss, no fuss.
Here is an example of a welded caps, for when you have a non standard size or an angle cut.
Attachment 12371