As you already know, a greenhouse kit is an investment and as such should be chosen carefully to fit your particular situation.   Below is a list of frequently asked questions regarding placement, size, etc. for greenhouse kits that hopefully will help in the selection process.

How do I determine placement for a greenhouse? 

Ideally, you want to do as little excavation as possible for placement of your new greenhouse.  You will need a reasonably level spot that is close enough to the house to connect to water and other utilities.  You will need enough space to orient the long portion of the greenhouse east to west, and have an unobstructed southern exposure with at least 6 hours of sun during the winter.

How much preparation will it take?

Before delivery of the greenhouse, you will probably need to do some leveling work on the greenhouse site, and then install a foundation to keep the greenhouse stable on the site.  Some greenhouses come with foundations, and if yours does, then obviously this step can be skipped.  If not, most greenhouse dimensions indicate "outside" dimensions that give you the foundation size. 

Basically a home greenhouse foundation  will consist of a square made of 4 x 6 lumber that won't decay, such as cedar, cypress, or treated wood.  Lay weed barrier over the area (if you use plastic, you will have to punch holes in it to allow for drainage) and carefully construct a square the size of the outside dimensions of your greenhouse.  The wood can be cut with a regular circular saw and joined together with lag screws.  Use a carpenter's square to be sure it is exactly square at the corners.  Once this is accomplished fill the inside over the weed barrier with whatever flooring material you want - gravel or sand work great, are inexpensive, and drain appropriately.  

Obviously you can also construct a concrete foundation, and if you prefer this, a drain will have to be placed in the center for drainage into a gravel pit below the foundation.  Concrete foundations should be 1" longer and wider than the greenhouse outside dimensions, and the slab should be no less than 3" thick.   It is always a good idea to fasten a 2 x 4 to the edges of the foundation with concrete anchor bolts to act as a buffer between greenhouse and concrete. 

How large should my greenhouse be?

This depends on how much you have to spend and how large your yard is.  The best plan is probably to get a greenhouse just a bit larger than you think you will need, unless space is at a premium.  You will find that you quickly outgrow the smaller greenhouses once you get started.  The general rule is the bigger the more expensive, so shop around until you find something within your budget and space requirements.  There are greenhouses available to fit virtually every homeowner's needs!

What's the best framing material?

Wood certainly looks nice but it is in general not the best choice for greenhouse framing, as it warps, cracks, and shrinks with time, humidity, and age.  PVC is economical, but it is not terribly good looking and is somewhat susceptible to high winds.  Aluminum is cost effective, requires little maintenance, is lightweight, and holds it shape, making it the material of choice for most greenhouses.  These are general guidelines only, however.  If you prefer wood,  kits these days usually use high quality cedar, cypress, or redwood, and will likely look nice and function well for years, and if you are on a budget, PVC will function fine as long as you give it a bit of shelter from the strongest winds. 

What's the best window material? 

From best to worst - glass, plexiglass, ribbed polycarbonate, and polyethylene.  All have advantages and drawbacks, however.  Glass lets lots of light in and insulates well, but is expensive and heavy.  Plexiglass also insulates well and lets lots of light in, but becomes yellow and brittle with age.  Polycarbonate resists harmful UV rays, though it lets in a bit less light.  It is also cheaper to ship,  but it is rigid and you can't see through it.  Polyethylene is malleable for following curves and is cheap to ship, but  needs replacing every few years.  Any two or more of these materials can be combined with a good result, such as glass on the sides and polycarbonate on the roof.

Having said that, shop carefully and find the greenhouse that best suits your needs while staying within your budget.  If you can only afford PVC with polyethylene for now, then go with it and be happy!  It sure beats not having a greenhouse at all!