Can I Install OEM Keyless Entry?
You can get the OEM keyless entry kit for around $200 for 1991-1993 model years. 1994 and later cars require a $350 kit.
Installation of either one is quick and easy. See NSX FAQ DIY: Installing OEM Keyless Entry for Do-It-Yourself information and an explanation of the difference between the two kits..
[BCH] It’s $436 for the ’93+ kit plus around $60 or installation (that’s what they told me). Apparently if you call the parts department and haggle they’re willing to sell it for $250 and install it yourself.

What Security Systems Are Recommended?
[CA] I have installed the Clifford Intelli Guard IQ 800. It is worthy of the NSX. Mine has remote entry, remote trunk release, perimeter sensor (keeps touchy-feely people away) and is very flexible. And more importantly less costly than the stock remote entry (@ $500.00 installed) and has a $3000.00 theft refund. I had an Alpine with the same features, I like the Clifford better. It also has a lifetime guarantee, even on the remotes; Alpine does not. Shop around, ask questions.