I'm not sure about which router would be the best entry level with all the features. I got my experience in our lab and at work where I've used a wide variety of the ISR G2 line (2900, 3900 series, etc.). I'd think if you could get access to an 850 series or higher with security license it should cover the features you need to practice on.
The exact number of questions varies according to the pool you draw from but the CCNA R/S and CCNA Security are about the same length. I took and passed both this year.
You may get a whole lot more response on this subject over at the Cisco Learning Network. They have a CCNA Security discussion forum:
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...