Could it really be about anything else?  I am a garage specialist, so of course not.  

First, let's get this out of the way.  Single car garages are made to store lawn tools and maybe a motorcycle.  Two car garages are the minimum an auto enthusiast would consider.  Three car garages are starting to get there.  When you have indoor parking suitable for six or more, you are in the zone. 

Let's talk about attached vs. detached garages.   

I recommend both (why can't I put a smiley here?).  But, seriously, having both is great.  In fact, I'd call it essential if one is going to do much serious work.  From both safety and comfort standpoints, the detached garage is better for hard working spaces.  Air compressors and tools make a lot of noise.  Welders and grinders make dust and sparks.  None of these are things that really belong in the house.  Also, there are increased fire hazards from welding, plasma cutting, grinding and using torches.  

But, the great thing about an attached garage is the convenience.  It's right there.  No walking in the rain to get to the garage.  So, I recommend that enthusiasts have one of each.  The daily drivers and the lawn equipment go in the attached garage, and the play toys go in the real garage.  All of the heavy tools and heavy work are kept away from the house.  

What about style? 

For resale, or to keep from running afoul of the Home Owner's association in some communities, it is generally better to keep the architectural style of the garage similar to the home.  While there are some exceptions, having a similar style keeps the flow more natural.  This is especially true when the detached garage is close to the house.  If they are not close, then it doesn't matter nearly as much.  However, a steel building will not add as much value (or cost as much) as a matching brick or sided building.  

What does an enthusiast want in the garage? 

This is a VERY tough question, because different people will have different usages planned for their space.  Here is a list of possibilities:

Lots of power available, both 120V and 220V.  If there will be a lot of fab work, 220V might be needed for an air compressor, welder, plasma cutter, lift and other high power consuming tools.

Space.  We all need space.  Floor space, loft space, attic space, shelf space... getting the idea?

Light... lots of light.  

HVAC.  It gets hot and cold out there.  Insulation, too.

A man door.  Opening a large garage door to walk in and out allows a lot of heated or cooled air to escape.  (I didn't make up the name, and women are allowed to use the same door... a standard sized door is referred to in the biz as a man door)

Nice floor.  For those not welding or torching, an epoxy floor might be perfect.  Welding a torching will kill that type of floor, but the concrete should still be sealed.

Air Compressor shed.  Those things can be loud.  Having the shop plumbed for compressed air is a great bonus.

Plumbing.  At the very least, there needs to be a sink.  A bathroom with a toilet is also handy for a detached garage.  A shower would also be nice. 

A seriously beefy workbench.  And if it has enclosed storage, that is a HUGE bonus.  It makes clean-up a breeze.