So, I might not worry about named bullet potions, but I might invest into some Heroism potions (simply cause they make things easier).
The formula on the Potions SRD page states: level of the spell × the creator's caster level × 50 gp.
So 3 x 20 x 50 would give me a total of 3000, however, Heroism is a 3rd level wizard spell so a 3rd level wizard could craft the potion for 750gp (same effects duration of 30 minutes). If I wanted a level 20 to make it (for a duration of 200 minutes [10min/level]) it would cost 3k?
Am I bound to buying potions at their lowest possible level? Do I still need an alchemist to make this stuff work?
I've purchased all my equipment for this level and have 25k left over, so potions that make my life easier seemed like the way to go.
If you're not an alchemist there really is no reason to get high level potions (unless you absolutely need high level magic fang or barkskin).
It's generally MUCH cheaper and more efficient to buy multiple copies of the same potion at min caster level then it is to buy 1 potion at max level.
Greater magic weapon etc it makes sense because higher caster level does not only increase the duration but also the effect. At first it's a +1 then for every 4 more CL its another +1.
If all that is modified is the duration, like with heroism, then its usually better to buy them cheap. Why?
50 min heroism (CL 5 minimum) - 750 gp
200 min heroism (CL 20) - 3000 gp
As you notice 4 times the duration for 4 times the price. Seems fair, right? Yes but perhaps not smart. Would you really gulp down 4 potions in those 200 minutes? Or would you drink 1 and then maybe not another one for the rest of the day because there's no fight anymore?
Once we get into the area of 1 hour/level spells it can make sense agian, so that you use 1 potion and are set for the entire day. But even then using multiple lower levels whenever useful is probably best.