Eric,
I just finished inking and marking my new Bearcat. For years I did all of my marking with Rapidographs. Now, I've all but given up on them. Large art supply stores here in Dallas still sell them but the ink supply is drying up literally. A couple of days ago, I paid $5.06 for a bottle of silver ink for drafting pens and found it virtually unusable.
What the art supply stores seem to feature these days is a large assortment of purpose made pens. I have tried a number of them. I suggest you take an old painted flap or whatever and try the pens you like. You should be careful about several characteristics when selecting a pen. Line widths are sometimes marked, usually not. Gels may never dry and will continue to smear until you get the clear on. Some inks won't come off when you goof. Make sure that whatever pen you use can be removed with alcohol. Some rapidiograph inks even don't remove cleanly.
Here are some pens you might try.
Prismacolor PM-98 Black, fine and bold tips. The fine tip is about .040. This is a litle broad for most but I was inking black on dark blue. It goes well on glosy surfaces, dosen't smear, and cleans up well with alcohol.
Prismacolor PM-118, Silver, also a broad line when held vertical but will draw a finer line if held at an angle. Both prismacolor pens draw well on glossy surfaces. The silver ink dosen't smear and cleans up well from most surfaces and colors. I had to do a bit of scraping with an xacto to get the last hint off of white Superpoxy. Pismacolor pens are available in a very large range of colors.
One of the best is available almost anywhere. It is the Sharpie Ultra Fine Point Permanent marker. Draws a very fine line, probably about .4 or .5mm, draws well on glossy surfaces, dosen't smear and cleans up well with alcohol.
Some Uniball pens work well. I tried two nearly identical pens in silver, both marked well, one smeared, one didn't. You gotta be careful
Staedler Pens are inviting as they are marked as to line width but smear.
With any paint or inking job there will be mistakes. Sometimes you can clean up a paint line with ink. It is always possible to scratch off or clean up a small spot with an exacto knife. It leaves a dull spot but you are going to put clear on anyway.
If I were getting started, I'd find it a lot easier and cheaper to pay $2.69 each for a couple of pens instead of investing in a mechanical drawing set. I'm using purpose pens more now and am about to give up on my rapidiographs.
As for lettering, I just tried new computer cut frisket stencils from AJT Laser. They are great. Kyle has many fonts and types anything you can imagine for the stencil. I used 1/8" letters, military font on my new Bearcat. After the backing is removed, the clear frisket stencil is positioned, (may be repositioned), and paint applied. Even with epoxy the stencil can be removed immediately to leave a perfect marking. The frisket plastic has a very low tac adhesive. I didn't try to reuse any of my stencils even though I used 10 "push" (per Grumman drawing). I don't know what Kyel charges for these stencils (I haven't gotten my bill) but you can talk to him about your needs at 817 831-1405 in the afternoons. This has got to be the absolute best as compared to trying to use rub on marking with each letter applied indivdually with possibly uneven spacing. I used about 40 stencils ranging from "push" to "Fuselage access" to "static port, don't block". They all went on in one fun night, in epoxy, with no Boo Boos. Other than the frisket, I used a piece of 8 1/2 x 11" tppewriter paber with a hole in the center for masking tape outlining the frisket. I used an airbrush and no other masking with no apparent overspray.
For what its worth.
Al