Since you've researched these and are somewhat familiar with them, and have decided on the fuel/heat type, I think your next step should be to decide just how large of cuts of meats you plan on smoking and the length of cooking time involved as it directly relates to adding fuel and ash removal and/or ash fall-out from the coal grate. Depending on the size of the cut and size of the cooking grates, a vertical smoker such as a kamado can be somewhat limited to what you can smoke in it, such as full slabs of spare ribs, large whole beef brisket and large 7-bone whole beef rib (prime rib, rib eye roast). That said, if you don't plan on smoking full/large cuts like those, the Kamado-style cooker may be your best choice due to efficiency (less fuel use) and possibly easier temp control than the horizontal offset. You can still fit small/med-weight whole brisket and half beef rib-eyes with ease, as well as full slabs of ribs if stood on edge and formed into a ring on the cooking grate...so there are ways to get around some of these size limitations.
Offsets do have an advantage of being able to remove ashes easily with a small fireplace shovel or other makeshift device to scoop them out, while some vertical smokers do not have this ability, so mods can be done to raise the fire grate for more ash build-up over longer smokes, etc...just some things to consider, and maybe ask your friends how well theirs handle it, and what they done to make improvements, if anything need changing.
As far as mods to smokers, I have not used any charcoal smokers that couldn't use a little help in their performance, so yes, expect to do a little tweeking as you learn how it likes to run and what you would like it to do better. One of the typical mods for horizontal offsets are baffle or tuning plates under the cooking grate near the firebox. Another is to lower the exhaust vent pipe inlet to cooking grate-level with a down-comer. Anything you do for smoke chamber mods should help to even-out grate temp variances and/or improve overall temp control.
Hope this helps you decide which direction to go next. BTW, if you're looking at a $300 limit for smoker purchase, and if grate size is comparable to the kamado-style cooker you're looking at now, you may want to look into the 18.5" WSM (Weber Smoky Mountain). List price is $299, and they can be had for less if you hook-up with the right place at the right time. Lots of happy users here on SMF, and they are built to last a lifetime. Prices should be coming down a bit over the next couple of moths with some venders, so keep your eyes peeled. Also, check Craig"s List...great deals can found if you look often enough.