There are a number of pen/pencil kits out there that use the excellent "Schmidt" pencil mechanism. These are much, much better than the click type for the slimline style or the screw type that are available from Berea for the Euro and Am Flattop style (must refill a lead from the front and this type of mechanism is really crap). The pen or pencils that use the Schmidt type are:
Perfect Fit convertible and El Grande Ball point pens,
both are from Berea and can be easily converted from pen to pencil by just changing the refill (you can have the mechanism on hand in case someone wants a pencil). The Gent Jr from CSUSA comes also as a pencil that uses this type of refill, but it is a dedicated pencil kit, whereas the two earlier mentioned are convertibles.
The Schmidt mechanism are great quality, I use a pencil with this mechanism for quite some time and never have had a problem. My experiences with the other pencil types are not very good. I had about half of the ones I sold returned to me due to failure of the mechanism, so I pretty much gave up making pencils.