A fortepiano is a major investment; the majority of musicians keep their first instrument for their entire life, even if that was not their intention at the time of purchase. Therefore one should go about selecting an instrument with the appropriate care.
There is no such thing as an all-in-one fortepiano; between 1790 and 1830, south-German and Viennese pianos changed and grew as rapidly as have computers over the last several decades. Therefore, any one instrument will always be better suited for certain literature than others. Modern fortepianists - just like modern harpsichords before them - have begun to realize that they will need at least two if not three instruments to properly play the full range of Classical piano music. The idea that one can play Mozart, Haydn, Beethoven, Schubert, and Schumann on one and the same instrument is just as unwise as the notion that one can play everything from the Fitzwilliam Virginal book to Rameau on a French double harpsichord. Thus, when buying your first instrument, rather than trying to satisfy all your desires at once, it is much better to focus on one body of literature and buy an instrument well-suited for it; hopefully, latter in life you will be able to acquire more instruments to broaden your palette. In making a choice, two factors will limit the possibilities:
The second most important thing a modern musician must ask himself before choosing an instrument is:
"Why am I doing this?"
Each person will have different motivations, and each will feel more or less dedication to the idea of "turning back the clock". The world of historical pianos is like a foreign country; some may view a fortepiano as though it were a second home for weekends and vacations, while others will want to move there, master the language, learn to cook like the locals, and partake in traditional life. Using 18th century terminology, we might call the former Fortepiano Liebhaber and the latter Fortepiano Kenner. If you don't decide which one you want to be, you may wake up some day and realize you bought the wrong instrument!
Für die Liebhaber
If a profound dedication to the concept of "Historically Informed Performance Practice" is not your major motivation, then the modern fortepiano market is completely open and your selection process relatively easy. A number of makers produce instruments which may be quite fine in their own right but are of questionable historical relevance. There are a number of "generic" "Walter" or "Graf" pianos on the market, instruments made by mixing aspects from a number of different originals - though usually one or another of these "sources" will be cited as the instrument which the "copy" is "based on" or "inspired by". Other builders carry the process one step further and incorporate elements completely foreign to the original instruments, either of their own invention or aspects borrowed from later instruments, often altering the touch or sound to be more appealing to modern pianists. If such historical aberrations are of no major concern to you, you need only seek an instrument which you find appealing and affordable.
Für die Kenner
On the other hand, if you are seriously motivated to rediscover all the musical advantages (and limitations!) of the original instruments which inspired (and frustrated!) Mozart, Haydn, Beethoven, and others, then your task becomes much more challenging - though ultimately (in my opinion) far more rewarding! Choosing this path means that you embark upon a journey of discovery and growth, a journey which ultimately may last a lifetime; it requires both curiosity and determination.
Following this route, be prepared for surprises! Your musical and technical habits and expectations are more likely to be confronted than appeased; for this reason, you should neither be seduced by the siren song of the first instrument you find with an appealing sound and touch nor should you be put off by an instrument which sounds strange or is difficult for you to control. Remember, the instrument which immediately sounds and feels comfortable will most likely teach you very little in the long run. Keep looking, testing, and listening.
To determine the degree of historical veracity, there is no alternative but to ask questions - as many as possible! Do not be afraid to question the builder about any aspect his instruments. The majority will be honest and forthright about what they do and why they do it; avoid builders who are vague or evasive, or who try to turn the discussion away from the instruments themselves and more toward the recommendations of famous musicians who have played or bought their instruments. A low price may garner a large number of "satisfied" customers, but such popularity says absolutely nothing about either historical faithfulness or the quality of construction and materials!
Above all, always view the recommendations of famous musicians in the proper light. Never forget that each and every musician - famous or not - will inevitably recommend what he or she finds attractive, for a multitude of reasons. Whether or not these same criteria will apply to you is impossible to determine. If you let others decide for you, you are just as likely to end up with an instrument you dislike after a year as one which continues to satisfy for a lifetime.
Makers who are truly dedicated to helping the musician find the best solution will invariable be happy to assist you in your quest; by contrast, those who are merely interested in "selling" will only try to convince you that you have (always!) come to the right address. One good test is to ask each builder which other makers he can recommend, and why; if a builder can't recommend at least 2 or 3 colleagues, chances are he's more interested in closing a deal than in helping you find your ideal instrument!
It's always a good idea to take notes while visiting or talking to each builder so that you won't forget things later. Don't be afraid to go back to one builder if, after talking to another, new questions arise.
Good luck, and have fun in your process of exploration and discovery!