I own an old Commander twin and here's my story:
I also wanted a 337 (great aircraft), but then I found my old 1953 Commander on Ebay and ended up being the only bidder. Not what I had planned, but I'm glad she found me.
Everyone thought I was crazy as I was low time pilot and didn't even have my multi rating yet. A big twin as your first aircraft? Unheard of. And when I got my multi, nobody wanted to insure us. They hated me, they hated Commanders, they hated her age. So I flew her about 50hrs uninsured. But then I finally called up AOPA insurance brokerage and they placed it with an underwriter almost immediately. All they wanted was 15hrs with an instructor and that was it. Done. The trick is to not get hull insurance, then they don't care as much anymore. I know this feels weird, but on an old airframe like this I don't give a d*mn if something gets bent - either one repairs it (plenty of spares around) or parts it out. You will have recouped that hull insurance premium in no time. On a newer plane - then it's another matter. I pay $2000 year as a low time multi pilot. That's pretty cheap.
As for ownership - I'm not going to say it's all been smooth sailing with her maintenance, but all considering, she's been pretty reliable. She's only left me stranded twice and we've done some serious traveling. Actually, I'm writing this from a hotel room in Nashville, where we flew to from LA earlier this week. I put over 250hrs on her last year and my costs were roughly around $220/hr. Sure, she's got a ratty paint job and interior, a panel that used to be horrendous (just upgraded it) and very basic, but she'll out-climb, out-range and out-speed any fancier glass cocpit Cessna 182RG for roughly the same hourly cost.
Here's my one cent: if you want a twin eventually, then get it now and upgrade it and mold it into what you want. In these depressed twin times, buying an 'interim' twin will only end up costing far more as you will have to sell it at a huge loss. Same goes for an interim single, although their market is slightly healthier. You can get into a Baron for the same money as a higher end single, so just go for that. You'll be glad you did.