Hello,
In response to your current printer, have you changed your settings to lay down the most ink? The printer doesn't know what you're printing so it's going to print with it's standard settings. Most Epson printers have a "Ink Jet Back Light Film" setting made for transparencies. I would check around in your printers settings to see if there are any transparency settings. Also, increase your dpi to the highest it allows, as that will lay down the most ink. You should NEVER need to double or triple layer ink-jet positives unless you've got bad film and worse ink.
As far as the Epson R1800 goes, it's a great printer but it's been replaced with another model the R1900. They sell for about the same price. Just so you're aware, there is a difference between what inks your printer uses and what film you can use. The Epson uses Pigment based inks, which means you can only use waterproof film. You can buy dye based ink for the R1800 (and probably the R1900 now) from US Screen "Fast INK" and a few other brands offer their own dye based ink, but that will allow you to use non-waterproof (less expensive film) in your printer.
Personally I think dye based film isn't that great. I've always used waterproof ink-jet film with dye based ink in my Epson 3000 and never have ever had an issue shooting a screen. Unfortunately they no longer make the 3000 either, but if you could find a used one on craigslist or eBay I would get it. It's 16.5" wide printing and the R1800/R1900 is max 13x19 sheets.
As far as prints being opaque without software, that comes down to your knowledge of graphic software. I do not use a RIP for my epson 3000 and have printed multiple halftones and four color process jobs. If you can learn the "ins and outs" of your printer settings as well as how to properly adjust artwork, you shouldn't need the RIP.
RIP software is really only necessary (in my opinion) if you are doing that many halftones that you can't take the time to tweak the artwork. For standard 1-4 color jobs (not halftones) you probably won't need the RIP.
Just my two cents, hopefully this helps you out. Check your printer settings on the Canon before you invest in a more expensive printer you might not need.