Interpose is an old game that I have played through some times. Its a sidescrolling shooter where you collect money and upgrades to buy and sell stuff in shops and between the missions to improve your ship. Its a great game. Here is a video of it.

And just listen to this soundtrack from the CDROM. Its freaking awesome!
The main menu theme is a nonvocal version of the song that in my opinion is even better.

You should add it to the GOG games catalouge. :)