It is clear that your strategy is well thought out. And that you are looking at all of the issues in terms of the value proposition, short and long term equity position, future market strength, etc. Buying a home is an emotional decision wrapped around logic. So you and your husband are pursuing this the right way. The vacant lot certainly adds a bit of uncertainty. And there is absolutely no way to forecast what will happen.
I have lived in Allen since 1990 when the only thing West of 75 was the Mcdonalds and the Chevron...and farmland...all the way to Custer. And you can see what has happened...explosive growth. So we can only make our decisions based on what we know rather than what we think will happen. I know that probably doesn't help much. But there really is no right answer for the question. There is one thing that I can suggest and that is to contact the City of Allen zoning and ask if there are any zoning change requests for that land. Also, you might consider contacting the owner of the property and just ask them. These are the kinds of things that your agent can do for you if you decide to be represented.
As far as an interior lot, builders price position, etc...a builder will always negotiate in spite of what they are telling you. An experienced buyer's agent knows what the builders margins are and is trained to negotiate for their client against these margins. Your commitment to be in the home "longterm" is the best way to build an equity position and insulate yourself against the market. But longterm is the key. When the "unforseen" happens...especially now...it is almost impossible to protect equity unless you are considering a large down payment.
There are a lot of factors at play here of course. And obviously the more information you have, the more informed decision you can make. I would highly recommed that you at least speak with a buyer's agent, lay all of this out and get their opinion. I have several very experienced buyer's agents in my office that are very familiar with Allen and StarCreek that I could suggest to you. And you might consider speaking to several...just to compare notes.
I hope this helps some. Thank you for using Trulia Voices and please do not hesitate to contact me if I can help you in any way.
Alan Wynn - Branch Manager
Coldwell Banker Residential Brokerage
17101 Preston Rd., Suite 110
Dallas, TX 75248