WRT cylinders.... I've been a Praxair customer for 20+ years, and my lessons learned are: 1) get ones big enough that you aren't going back yearly and 2) get ones small enough that you can lift yourself. A lot of places will only put cylinders on the loading dock for you now--no assistance into your truck allowed. I've downsized both my Argon and StarGold (CO2/Argon mix) over the years as I aged so I can still handle them myself.
WRT to homemade carts....Pick your bottle first, then build the cart around it/them. It's dangerous to undersize your cart such that it can tip over and let the bottle(s) fall. Carts need to be stable so the bottle--even when chained in--won't tip when you bump them...which you will. Commercially made carts limit the liability by engineering the tray (base) such that an oversized bottle (they are mostly standardized sizes) can't fit in a smaller cart. But some redneck always seems to modify them after too many beers.
One last tip for carts: don't cheap out on wheels, specifically, the size. With gravel, metal parts, etc., in your shop, using too small wheels will be frustrating as the cart will hang up on every little obstruction.
Good luck. Post pics please!
When you are ready to tackle the table let us know here as well. Lots of great advice is available.