I started off doing a conversion. I think the 2 biggest factors to be concerned with are time and money. If you are short on either - convert. Within 2 hours of bringing my first machine home - I had it gutted, cleaned up and ready to start repairing/painting/building. I was able to finish it in 5 days. Here is what it ended up looking like:

I learned an important lesson with this arcade. Hopefully this little tidbit of advice isn't lost on you.
You can't do it all with a single machine! My first attempt I tried to mount 2 sets of 8 way controls. The CP just wasn't wide enough. Now this machine only handles 4 way games in a vertical format. This machine cost me over a $1,000 because I had to buy all the tools.
My second machine was a convert from a 1976 Lemans. It turned out like this:

It's a very clean conversion.
My 3rd machine was a jukebox which I built from scratch.

Not very smooth lines, but I actually need every bit of storage this box provides. I built the entire cabinet from my own plans. Quite a bit of time is spent designing the machine.
My last machine is a half and half showcase machine. I built the TV stand and converted the pedestal portion from a Combatribes converted from a Xenophobe. Here is a pick of it nearly finished -

To be honest, building or converting are near the same as far as I'm concerned. I showed you the pictures because I've done a little of both and although there is pride in saying "I built it from scratch" there is also a bit of nostalgia saying "This was a 1976 Atari LeMans game".
If you don't have the time/money to build a machine exactly how you want, convert something. Don't make the same mistake many people do and try to get 1 machine to do everything.