RPM grimoire only

[fifiste]

I have an idea for a magic system that at least first sounded rather self consistent in my own thoughts. Still I would like some feedback and have also a few questions.
Idea .... Use RPM as in Monster Hunters - with exceptions. You can in reasonable time cast only pre-defined spells that are in your grimoire already.(maybe treat PC-s as adepts in these cases and non-adepts in other cases).

For adding new spells into grimoire you need to do it as an invention - with thaumatology as the invention skill.

The idea here is to remove some of the on the fly adjudicating troubles form the GM and to remove time spent on figuring out spell parameters on the fly. It can help the players more prone to decision paralysis and especially the other fellows on the table.

And i think in some games it can give cool focus on the research and development angle and on some others vice versa remove all this from sight(keep it DF-ish) and just use spells bought/looted/found etc.

Questions:
Should I just use the costs on the inventions chapter?
What would TL-s be? - the complexity would probably be rather high for most spells?
What would be the cost at character development to have some list of spells already in your grimoire ? Figure out their retail price somehow and buy them for cash/points for cash or signature gear etc.?