Well, you did everything in your power to do well financially, even maxed Merc, so the game rewarded you by giving you more money than you know what to do with. Not saying the finances in the game are perfect (they are OK), but I think your situation makes sense from a design perspective.
The game difficulty goes up in the final stage of the game (and when I say final stage, I mean the final 20% of the game). I think you didn't mention going to Deep Caverns, right? It is quite tough.
If you do not want to try ironman, I do not know what else to suggest. (btw, I do not find that the game has been designed with ironman in mind, unfortunately. However, some folks play it ironman). Maybe try another build. Sniper+pistol is quite powerful, but the point is to find the builds that you have fun with.