"most of all good customer service in the event that there is a problem with any of the parts and pieces in the kit..."
Just for the sake of another viewpoint, this could be a bit controversial with these engines in particular. When I built my first inframe kit & started participating on this forum, I too recommended the above as it's wise advice for most products, vendors that support their products must be good ones right?
There's a couple of catches though, ones almost unique to this product and those that sell them & that's overall cost and your convenience.
Most of the factory issues with these engines are defective/damaged components and poor assembly - the assembly issues you'll need take care of yourself regardless of warranty and the vast majority of defective or damaged components are simple and inexpensive to replace, a carburetor can be had for a mere $12 for example.
So the question arises, is it really cost effective to pay extra for a warrantied engine that may be of no better quality, from a merchant that charges more for the customer service they're known for - to then pay for the return S&H, wait for it to get there, wait for it to be "approved" for replacement and then if it does, pay the return S&H and wait some more... when you could have just bought a new carb or whatever?
When the parts are so
cheap that S&H alone can exceed their cost, you have to wonder if it isn't far more cost effective to simply ignore the warranty and fix it yourself.
So what about something more serious? What if you get a terminal lemon, one that just detonates on first start up or gimps around making horrid knocking noises, one that's definitely not long for this earth? Well, that's
when the warranty returns are most disputed by the vendor - if it's just a little part you'll probably get it, if it's the entire engine it's far more likely to be judged "operator error", "incorrect break in procedure" the "wrong oil/mix" or whatever else seems "likely" to them - many won't accept a return on an engine that's modified in any way, mounted or even just been run... which effectively means the only problem that might be covered at all is obvious and grave S&H damage.
I don't mean to infer that all these merchants are deliberately misleading you to get your money - some may well intend their customer service to be good, to establish a reputation for support... the problem is they do have to charge somewhere to recover their costs, to pay for that support and the question is, is it really worth it to you?
My first two inframe kits I purchased warranted from "reputable vendors" - I eventually realized that all
the issues I had were minor, too minor for the return & wait process and that even if I did get a lemon I wasn't going to bother to return it, these engines are so cheap I'll just get another & use the dud for parts, those parts even cheaper as I got them as a complete engine.
So now all I do is find the least expensive kit retailer on ebay or whatever, one who has a good reputation - not for warranty or service but for just getting their product to you in a timely fashion and the product is as advertised.
Interestingly enough not only has this saved me hundreds of dollars, the engines themselves were some of the very best I've had.
By all means get a warranted kit if it makes you comfortable, if you feel a need to support a favorite vendor, if it's your first and you want to cover your bases - but know it won't guarantee any better quality and to be blunt with an engine as simple and cheap as these, you're better off on your own, they're all a crapshoot and you'll need fix it regardless.
Might as well skip the time consuming & costly middle man games and just get another