I'd be inclined to carry on using the OM1 with the winder, if you want to stick with 35mm. Or look at other brands for a larger camera body - the original Pentax K series bodies are pretty chunky and still give you full manual control, big viewfinders, etc.

If you want a larger camera which will improve the image quality then go medium format and the largest frame size you can afford (different cameras can produce 6x4.5, 6x7 or 6x9 frames on the same 120 film).

The Leicas just seem to be a huge amount of money, and I'm not convinced that they're enough of an improvement over the other recognised brands to be worth the extra you have to pay. I can find a minty Pentax KX or K2, have it CLA'd, buy two or three of the best lenses available for it and still spend less than a Leica body with a single lens. Those two are every bit as long-lived and robust as any other brand providing that you don't abuse them and have them properly serviced once in a while.