You are asking at least two questions:
- How important is frame geometry?
- Should I buy a complete bike or build one from the frame up?
1) Geometry is *very* important. Geometry on track bikes can become very particular. But, until you know what the different angles mean and until you start specializing in some sort of riding style (endurance, sprint) then you'd probably be best served with an a basic, neutral frame geometry like the Bianchi Pista Concept, Felt TK2, or 2008 Fuji Track Pro. Those aren't the ONLY one's by far. They are just a few with which I am familiar. Actually, MOST track bikes are sort of neutral.
If we have "neutral" bikes, then we have to have bikes on each extreme. For example, compare the Cervelo T3 to the Dolan DF3 (I would mention the BT Stealth here, but BT won't publish the geo). The Cervelo is an endurance/pursuit bike and the DF3 is a sprint bike. Yes, they can be (and have been) used for the entire range of events. But, they were designed for what they were designed for.
Important metrics that determine your riding posture and how the bike will handle (already assuming you are on the right sized bike):
- Head tube length (this will determine how low you can get with aerobars)
- Head tube angle
- Fork rake (this combined with head tube angle will determine how "twitchy" or "stable" the bike handles. Sprint bikes have short rake for quick maneuvering, TT bikes have long rake for stability when in aerobars.
- Seat tube angle (Endurance bikes are setup similar to road bikes whereas some sprint bikes tend to have a more upright seat tube for several reasons.)
Regarding components:
For the most part, when you buy the nicer components you are buying durability and reliability. They will work well and do so for a long time. I know one guy who has been racing the same campy cranks for over 20 years. I've heard similar stories about hubs, handlebars, etc.. A friend of mine races some handlebars that she got off of her dad's old bike.
On the other hand, LOTS of budget gear is really good. For example, Formula hubs are basic hubs, but are really strong and the bearings are replaceable...and they are A LOT easier to deal with than Dura Ace hubs which require constant checks to make sure that the bearing on the rear aren't being overtightened due to putting the wheel on/off the bike so often. I carry cone wrenches in my track sack.
As far as components go, I'd ask around and pay attention to what other people who have been around a while use.