but I am curious as to what other, more seasoned Busse collectors feel are the ultimate Busse knives to have in a collection.
MOAB
Killa Zilla
Ruck Kensai or ak47, or both. If it was me I'd have garth cut down an ak47 to one handed.
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nmfbm
cg ffbm
cg hogfsh
cg sjtac
BAD
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NICK set, both thin and xxl
pbf set, the killer b, 6" and 8"
a complete basic set - CRINKLE COAT version, not the smooth. including 3,5,7, and 9
any field grade you can get your hand on
thats just for me, going from the stand point of their closeness to "perfection" of design being their biggest point of reverence. The moab, killa zilla, ak47 and ruck are all busses that users and collectors dreamed about and never really imagined would come into existence. I'm not really a fan of the ak47 because it doesn't function well as a sword or a machete wich is why I'd have garth customize it. anything customized
in the custom shop is immediately more collectible because it has a feature no other one of it's model has.
The nmfbm is the biggest choper in the busse line that within the "normal human use" range. any bigger and you start getting into actual khukuri size which requires a different approach in use. The ffbm is something you need to hold to really understand. it's as close to a "sledge hammer knife" that busse has made. The hogfsh is truly perfect in every way. I would buy it, the basic 9 and the nmfbm over every other knife on this list. The sjtac is... it's amazing for it's size, the ultimate do all knife that doesn't get stuck in the "does everything poorly" category. The bad is the best slicer produced by busse in a production manner.
The NICK sets are awesome if you can afford to have them. they function well as kitchen knives, and were within the "high end kitchen cutlery" price range when they were first offered (287$ i think), but now in the 450-600$ range they are pretty high up there. The pbf's are among the tankiest knives ever produced by busse. They are so thick at the edge that they almost don't function as knives. The basic set is something every collector should at least handle for a while. They are so well designed and so simple, the basic 9 being one of the greatest knife designs I've seen in the time I've been buying, trading and using knives. Field grades are part of busse history and inherently worth having even if you don't like the design.
There are plenty of other rare knives out there, but this list represents the ones that stood out to me when I had them as being really stellar designs that were worth having both for looks, feel and balance, but also for their functionality.
Also, do you recommend buying the new Busses as they are issued or just looking for particular ones on the second hand market.
Buy them as busse offers them if you want the knife. It's the cheapest way you are likely to get them, you might come into the market during a lull where prices are at or slightly below retail, but even at the best of times it will only be 10-20$ below retail. Certain models command a high premium, such as the ak47 and the ruck kensai. Both were under 600$ at the initial offering, now they command 900$+ even in a depressed market.
What do you think of other Busse knives such as the Swamp Rat versions? Again, this is for collecting.
There are no other busse knives. Swamp rat and Scrap yard knives are companies that are unto themselves that have the benefit of being able to talk with, get advice from, and sometimes use the same equipment as, jerry busse. As far as collecting, none of them are as high end as busse, they don't command the same price and they don't really have the same active market. as far as users, both are so close to busse in strength that few would be able to really tell the difference in use. It depends on what your aesthetic is, and what you want out of the knife.
Swamp rat and Scrap yard, being budget companies, both appeal to users much more than collectors.
Finally, how important is having the original Busse cardboard sheath when buying a knife for a collection?
It doesn't make any difference. I think the ideal would be to have a nice heavy duty leather sheath or a cordura kydex that fits it. The leather is nice looking for collecting purposes, and the cordura kydex is both functional as a sheathing method and doesn't wear on the knives as much as a straight kydex sheath does.
LOL! The main perpose of the cardboard sheath is to protect the blade... Wait... The main purpose of the cardboard sheath is really to protect your postal courier during transit! 
. The straight handle line is overrated. If you want to go for older pieces, go for the pre-straight handle knives, or the very first renditions of them like the proto-shba.
