Hi Kris and welcome to the next level . This is a subject that is near and dear to me .
What kind of kitless pens do you want to do , Ballpoint , rollerball , Fountain , hybrid ? Each has it's advantages and it's pitfalls . Kitless Ballpoints are my favorite because of the challenge of the mechanicals involved and can be as simple as a slimline transmission and a couple of 7mm tubes to completely kitless where you make everything but the refill .
Rollerball and Fountain pens can also be made many different ways , friction fit cap , snap fit cap or threaded cap . Friction fit are the simplest in design but you have to turn your parts to very tight tolerances to get a tight friction fit that won't open in a pocket . Snap fit are a little more difficult , I'm sorry to say that I really haven't tried this style yet so I'm not sure about the mechanics of them . I must try one of these someday . Threaded cap pens are easy enough to do if you have the right equipment like a metal lathe or a set of taps and dies . The metal lathe has a very big learning curve , which I'm working on now , to be able to cut threads , especially multi start . Taps and dies that will work can be purchased from many different sources , even a set from Harbor Freight can be used successfully however most threaded cap pens use something called "Multi start" threading , which is 2 or more sets of threads combined into what looks like one set of threads , This requires a special type of tap and die that are quite expensive and must be special ordered .
I know that there some who will disagree with me but I would suggest that you start with kitless slimlines . They are the simplest to design and build but can be quite striking when you get it right , like this .
This pen got me into the Penmakers Guild but it is just a kitless slimline in that it uses a slimline transmission and a Cross type refill .
Try this link and take a look at what can be done with a slimline transmission and some 7mm tubes
http://www.penturners.org/forum/showthread.php?t=36211&highlight=kitless+pen+challenge .
Also check out my photo album to see several different types of kitless pens . Also go to the Penmakers Guild and check out some of the most awesome work you will ever see
http://www.penmakersguild.com/browse.php