Ask Mr. Smarty Plants

You are correct that this is a complex question and there is no one source that can give you the answer.  We can, however, suggest some resources for you to use to research and answer your own question.  First of all, I'm not absolutely sure which designated ecoregion you are in.  You can use the University of Texas Bureau of Economic Geology Ecoregions of Texas map to determine that.  From your description I suspect you are in the Balcones Canyonlands part of the Edwards Plateau or the Floodplains and Low Terraces of the Texas Blackland Prairie region. Since the boundary isn't generally going to be abrupt, both will have plants that will have grown in your area before the neighborhood was developed. 

For online sources, the Walter Geology Library at the University of Texas has several publications available  that can help you determine the historical vegetational cover and biodiversity before development in your particular area of Central Texas and more recent studies of current coverage.  However, the average biodiversity per acre is probably not something you are going to easily find.

In Virtual Landscapes of Texas on the webpage of the Walter Geology Library you can search by subject (Botany-Texas) and find:

B. C. Tharp.  The vegetation of Texas : being the first of a series of brochures purposed to present the scientific scene with accuracy and interest.
Publisher: Texas Academy of Science
Series: Texas Academy publications in natural history. Non-technical series
Date: 1939

On page 3 is Plate 1, a map of Texas showing 18 different vegetational regions.   I believe your area would fall in Region 4, but you would be better at pinpointing it.  You probably need to look through several of the areas around your area.  Tharp's treatment is rather broad.

There are also two other texts that you can find in Virtual Landscapes of Texas by searching in the Subject field using "Botany - Texas."  They are the First Annual Geological and Agricultural Survey of Texas (1874) and the Second Annual Geological and Agricultural Survey of Texas (1876).  The botanical emphasis is on agricultural crops but there is information on trees and grasses that might be useful.

In the Walter Geology Library's Online Texts and Maps you will find:

Balcones Escarpment.  A guide to the geologic setting, flood problems, ecology, natural history, archeology, and land use problems of the Balcones Escarpment and adjacent areas.  Editors: Patrick L. Abbott and C. M. Woodruff.  Published for the Geological Society of America Annual Meeting in San Antonio, Texas.  November 9-14, 1986.

In this publication you will find the following article:

PLANT COMMUNITIES OF THE EDWARDS PLATEAU OF TEXAS: An overview emphasizing the Balcones Escarpment zone between San Antonio and Austin with special attention to landscape contrasts and natural diversity
David H. Riskind and David D. Diamond (pages 20-32)

This article also includes an extensive bibliography of publications (books and journal articles) that you might be able to find at the University of Texas at San Antonio Libraries.

On pages 1-15 of the Manual of the Vascular Plants of Texas by Donovan Correll and Marshall Johnston (1979) there is description of the plants of the various ecoregions of Texas as shown in the map cited above.   This volume is available at the John Peace Library at the University of Texas at San Antonio. 

Also available at the John Peace Library is Gould, F. W., 1975, Texas Plants--a Checklist and Ecological Summary. Texas Agricultural Experiment Station, Texas A&M University, College Station, TX.

Another publication, Edwards Plateau vegetation : plant ecological studies in central Texas / edited by Bonnie B. Amos and Frederick R. Gehlbach (Baylor University Press, 1988) is also available at the John Peace Library.   Within it are three chapters that should be particularly useful: 

• An Introduction to Environments and Vegetation by David H. Riskind and David D. Diamond
• Vegetation Before 1860 by Del Weniger
• Woody Vegetation of the Southeastern Escarpment and Plateau by O. W. Van Auken

Other chapters in this publication might also be helpful.

Finally, there is an account of a journey through Texas in the 1850s by Frederick Law Olmsted (the designer of Central Park in New York City and considered by many to be the father of American landscape architecture) that you can find at the University of Texas at San Antonio Librairies in various editions—A journey through Texas, or, A saddle-trip on the southwestern frontier : with a statistical appendix—first published in 1857.  In it are descriptions of the landscape and vegetation he encountered during his travels.

Good luck with your undertaking and thank you for your kind words.  We hope these sources will be helpful in selecting the plants for your project.