Let me join in Welcoming you to Shadowrun!
I Have been gaming for over 20 years, have played various incarnations of Shadowrun since 1st edition, as well as numerous other game systems.
Having recently started a campaign with players that have pretty much only played D&D (some for only a few years, some for many years), be prepared to try to shift the mentality. Some of your players may want to be the "heroes" and have trouble with some of the things that Runners do. They might also treat their runs as Storming the Tomb of Horrors.. go in, guns blazing, kill every enemy in sight, and loot every last valuable thing they can find. Impress upon them the need to be stealthy, get in and out with no fuss, leaving few trails behind, and not having the luxury to stop and rifle through the pockets of every downed foe.
As to your questions.
1) I would advise starting with SR5. It has the benefit of streamlined rules, and only a single Core rulebook at the moment. It's easier for your players to digest and get used to, and you can slowly add content as new books are released, and you have a chance to pick them up and decide if you want to use them. Yes, some express that SR5 is too limiting. Yes, PCs have limits built in, on how many Hits they can keep. This helps to prevent the one trick ponies. There is not as much reason for someone to try to focus on getting a single skill, say Pistols, to a 20+ dice pool, if they can only keep 6 or 7 Hits. Spread out and dabble in other things. But, on the other hand, every PC has way to ignore those limits as well.
Yes, there are some confusing bits that could benefit with some errata. But you say you've been gaming for over 20 years now. This book is no worse than any other game book from back in the 90's, and most situations that come up can easily be adjudicated o the spot.
2) While I would also agree that you should start with no (or very little) houserules, SR is very friendly, mechanics-wise to tweak Once you are familiar with the Core rule book, it is fairly easy to grab one of the SR4 books and import individual pieces of gear, to hold you over until the new versions are available. Say they are offered a job where they need some really big firepower. You can tweak their weapons. Maybe Mr. Johnson offers them an Assault Cannon that does 2 less damage, or has a lower accuracy, less AP value, whatever. I gave my group a van as part of their payment. It was beat up, so I reduced both the Body and Armor by 30%.
Also, because Sniper Rifles tend to be just as mobile as a pistol in a running gun battle, and far more powerful, my group has been been asking for a change. I think we are going to reduce the accuracy on all Sniper Rifles, so that without any prep, you can't keep that many Hits, but if you take the time to set it up, take some Take Aim actions, you can once again boost the Accuracy up to make it a weapon to be feared. So yes, weasy to tweak, but play it as-is for a while first.
3) As it is with tweaking the rules, It is also easy to adjust on the fly to account for more or less PCs in a group. If you have a villain that is supposed to be challenging, yet beatable, and the group is just having no luck, or is smaller than average size, you can just start rolling fewer dice, or reduce their Condition Monitor by a few. Or, if the villain is proving to be a pansy, or you have a larger than average group, then you can roll more dice, add another bad guy or two.
Also, it sounds like you are fairly new to Shadowrun as well. If so, there are several podcasts out there that you may enjoy listening too. All of these are free on iTunes, but some of them require you to visit their site to get all of the episodes.
Hiddengrid
http://www.hiddengrid.com/ This is an actual play podcast. They start in the 2050's era, playing through many 1st/2nd edition adventure modules, converted for SR4. In the more recent episodes, they made the switch to SR5, so if you listen to them all, you can note the differences between the systems
Neo Anarchist Podcast
http://neo-anarchist.podomatic.com/ Opti (the host) presents the history of Shadowrun in a fun to listen to format, givng you the down low on why things are they way they are, and certain key events that happened in the past.
Arcology Podcast
http://arcologypodcast.com/ This podcast presents information and reviews about some Shadowrun Returns missions, as well as information about being a Runner. They did a 4=part Anatomy of a Run, where they give advice on how to handle The Meet, some of what you should be doing for Legwork, the different styles of the Run.. whether you should leave survivors or not. And how to help keep Mr. Johnson from fragging you over when it is time to get paid. They also started a 6 part series on different archtypes. Where, if you are playing a Street Samurai, or a Rigger, what you should think about when building your character, skills, attributes, gear, play styles, etc. It is very informative.
Critical Glitch
http://criticalglitch.com/ If you want to know about Missions, this is the place. Run by two members of the Catalyst Demo Team, they discuss missions, convention gaming, they do interviews with some of the writers and developers at Catalyst. This is also an entertaining listen, and you might get some ideas from what they discuss their players doing at various games... physical adepts engaging a helicopter in Melee, for example...
Good luck Chummer, and welcome to the shadows.