One more thing not to forget is the warranty of the Auto gate and axtra remote controls.

Choose the design of the automatic gate wisely too.
Normal gate style (opens outwardly), long sliding style and Folding style.
Those sliding style one usually will have a small gate for you to open for people to come in. without opening the gate.

Not sure of the exact prices tho.
You can try yellow pages or ask a few friends for recommendation.

remote is a small problem ler...you can always get it duplicated.
anyway for SeeD's case...i rather agree with scorgio about just getting the people to do it.

My friend who is studying Electronic engineering did a house alarm system for my dad..a simple one with remote control...and the cost to make one end up more than purchasing a security set outside. Not to mention it comes with warranty and support too. DIY doesn't mean it'll be cheaper. it depends on the thing that you're building tho..because they mass produce it.

But i totally understand what SeeD meant..
well if time and cost is not a problem...and you're doing it as an interest, just go ahead and DIY it. It's satisfying to see the end result/ product. money can't buy it.