Choosing the right bike...and.....custom fuel tank

I'm about to embark on my first moto-bike project. I'm shopping around for the perfect bike to use. I'm really into cafe racing so I wanted to model it after a cafe racer.

Is there anything I need to know before sinking $500+ into a *****in' cruiser?

The bike I'm looking at using is the Nirve Skulls.
I like it because it has the motorcycle looking front fork tubes and a front disc brake.

It has an aluminum frame. Is steel preferred over aluminum? Does it matter?

Any advice...or warnings to a first time builder are greatly appreciated.
Thanks!

Oh yeah, one other thing. Anyone out there build their own fuel tank?

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If you are building a frame mount kit and the price of the bike isn't your first consideration:

The first thing I look for in a bike to build a motorized bike from is front suspension. I have had bikes with and without it and I won't ride one without front suspension anymore. Cruisers with front suspension from the factory seem to be fairly rare. I-paint found one I think called a Kettler. I built my favorite bike out of a Felt cruiser but I added the front suspension.

The next thing I look for is a rear wheel without a coaster brake. I like good brakes. I don't like the coaster brakes much and they make it difficult to discard the poor/dangerous chain tensioners that come with the kits. If it has a rear derailleur then the tensioner won't be needed at all. If I plan to use a SBP shift-kit then one of the internally geared hubs are my preference.

The third thing I look for is Disc brakes. Rim brakes do work pretty good too. Just not as good in the wet.

Usually you can get some portion of these elements in a bike but not all of them. You then have to plan on whether or not to add the rest.

Thanks Clotho,
The Felt is an aluminum frame too, right? Any problems with excessive vibration with a rigid aluminum frame?
If you take a look. Those Nirve frames are very similar to the Felts.

The only frame problem that I have had to solve on my bike was some flex in the bottom bracket. I changed my motor mounts and that solved the problem. The Nirve bikes are similar and I have seen the top tube converted to a tank on one. I think it would be a bit small however. It should be easy to find some front suspension forks for that bike that are motorcycle-ish like those ones.

I like the fenders!

Thanks for the advice!