Selecting a kit is a second most important decision a builder would need to make. The first is the decision to build one in the first place. This one is called "Go / No-Go" decision. Kit selection is a surprisingly complex process fraught with many pitfalls. Here, I have attempted to clarify this process a little bit. Remember, though there are a large individual variation and personal preference that will affect the decision process. I am certain I have missed some issue others may find very important. Like I said before, it's just one guy's rant.
First of all, for the sake of discussion, let me define, in my own terms, 'right kit' means in the order of importance.
Right kit is the kit that will be completed.
Right kit is the one you can fly.
Right kit is the one you want to fly.
Right kit is the one you will keep.
Meeting these, you'd have a better chance of satisfaction with your project. Each of these can be surprisingly emotionally charged especially since one of the primary charter of experimental aviation is getting what you want, the way you want it.
This first one may be obvious but it's not as simple as you might think. This is the explanation behind all those classified adds for uncompleted kit projects you see all over the place. Let me expand on this a bit. Here are the factor that affects your chance of completion.
Once completed, you need to be able to fly the plane you built. Obvious but there are some thoughts that need to go into this aspect when choosing a kit.
Law: Check with your local authority about the kit you are about to build. Aviation regulation can differ significantly between countries and kit that are legal in one country may not be in others. Some countries do not have provision for homebuilding at all. Do your homework, or you may end up with a very expensive garden ornament.
Experience: The law may allow you to fly your creations but that does not necessarily mean you can, at least safely. Nobody is going to stop you (legally speaking at least) from building and (trying) to fly your 'TrickSter 2000' straight after your PPL checkride on a C-152. Needless to say, that's a disaster waiting to happen.
Environment: So you finally finished your 'SuperJet 2000'. You are ready for testing phase. You found out to your horror that the longest runway they have nearby is 2000' sod. Oops! Should've checked into that one... While that is an extreme example but you'd want to make sure that you can do adequate testing of your creation. This may involve high-speed taxi, ground handling, or even water op if it is an amphib. Simply having a runway nearby you can barely take off at max performance usually is not enough.
This one sounds similar to the last one but addresses slightly different concerns. You can and you want is really a different issue. Do you enjoy low-and-slow? pulling high Gs? going places fast? bush whacking? The airplane you build should be a reflection of your flying lifestyle. Each homebuilt are bound to be as individual as there are different builders. Study the candidates thoroughly. Talk to people who fly them. Find out what they do with their planes. If that matches well with what you want to do, you have a potential match.
I have read and heard from a few people the old advice 'Look in your logbook'. The idea is to study carefully what kind of flying you have done in the past and use that as a guide to your flying habit. While there is a good point being made, relying solely on your past may end up limiting you in the future. If I look into my log book, my flights are mostly short hops and local scenics with just one other person. Given this, It may indicate that I only need a Cessna 152 for my mission profile. This is grossly misleading. Here's why.
The reason I never do anything more than local scenic might be that the local FBO has this rule that you pay for minimum hours for every day you have the plane out. This would make taking a long trip an expensive proposition. I've dreamt about flying to Napa, Florida Keys, New Mexico, New Orleans, Atlanta, etc, etc... But not in a rented old C172...
The point here is that your logbook entry may be artificially constrained by what is available at the FBO or the rules they set. If you build a plane solely based on what you have done, you might end up imposing the same restrictions on your activity you are trying to get out of by owning your own plane.
This is particularly an issue when you are relatively early in your flying career and trying to select your dream plane. You just got your PPL and all your flying hours have been in the club's old C152. Chances are, you know that one plane inside and out. You know how to jiggle that radio knob just right so that it won't buzz on you, or how you have to hold left aileron up to keep straight and level. But you don't have any experiences with anything else. Heeding rule #2, should I stay with most docile and boring of the pack? Well, not necessarily so. First of all, you normally have years before the kit is finished. In the meantime, you have plenty of time to get into more advanced and bigger planes. Maybe you get checked out in taildragger, maybe high-performance / complex, or perhaps an instrument rating. If you just built that 'Easy-Baby 2000' by the time you finished it and flown it off, you may be yearning for that 'MasterCruiser 3000'.
There is a bit of educated guess involved in this. You will need to anticipate where your flying will be when you expect the kit to be finished. Ideally, the kit is versatile enough so that it can grow with you for a while. One of a quality of a good design is one where you can get into relatively easily and one you don't grow out of too quickly.