If you run test long term, you will shut down your natural production. Then you will depend on it, just to be in the normal range. This will be a life long commitment and can come with it's own set of side effects. For instance, you can become infertile. That may not be an issue now, but you are 20 years old. I'm sure you've experienced enough things in life to know that you cannot predict what you will want in 15 years. 450nl/dl is certainly on the lower end of normal, but if you can maintain this level between cycles, you can make good progress without needing to be on year round. I would suggest that you start by cycling test on and off, before you commit to long term use. 10-12 wks at 250-500mg EW, followed by PCT and then time off to regain natural production. Along with test, you can use an AI like arom at 25mg EOD to control estrogen Or adex at .5mg EOD.
As for your abs, it's all going to come down to diet. If you are 160 lbs now at 5'9, you have a lot of eating to do and it may require losing your abs for bulking phases. But you don't have to get fat in these bulking phases. Just add enough of the right calories to make the most progress in a given period. Then diet the fat off later.
Over time, as you get bigger and have gotten used to using a given dose, yes, you will get used to it. Gains will slow. Thats why coming off is a good idea. Eventually though, you will need more to make progress.
As for your HCG, I would start around wk 4 of your 10-12 wk cycle. I would inject 250-500iu 2x wk through the cycle and 2 wks beyond, up until you start PCT