Nice design, If you want to save some money you could use 1515 for the bed seeing you are only using the 1.5 side and not the 4.5 I dont think you will loose any strength by doing this. Also if you have a drill press and a mitter saw you can save a bundle on all the joining plates. I use acouple of different sources online to get flat bar aluminum in .250 inch sizes and then just glue a layout onto the metal and cut and drill. Last but not least you can make your own t nuts. They sell the blanks on ebay for a few bucks and you can make any size thread that you need. I have made 300 M3x.50 tnuts and 200 5/16x18. It takes time but after awhile it goes quicker and quicker.
Good luck with build!