Enhancements are an important and easy way to boost your physical and mental abilities in Reno. They're the difference between a regular Joe walking down the street and the Player Characters; they give you an edge on your opponents that may mean the difference between life and death.
You can get only 1 enhancement per level. If you do not get an enhancement in any given level, you cannot make up for it by getting 2 enhancements the next level.
For example, Ricky decides to spend all of his first-level money on weapons and armor. Ricky doesn't have enough money to buy an enhancement, so he doesn't get one at Level 1. Now Ricky is Level 2 and gets an enhancement. Despite the fact that he is second level, the maximum number of enhancements he can have is 1, because he didn't get one at level 1.
This does not mean 1 enhancement of each type; just one enhancement total. You cannot get a Strength enhancement as well as a Tech enhancement in the same level.
HP Boosts do not follow this pattern; you can get up to 4 HP Boosts total per character, there is no level restriction on when you can get them, and they do not count against your 1 enhancement per level restriction (i.e., you could get all 4 HP Boosts all at once and still get your 1 stat enhancement in the same level).
It is a smart idea to get an enhancement every level. It is not necessary, but it will give your character every edge to survive in the harsh world of Reno.