1) If just doing slipons without air filter I think you probably would be Ok but if you get any pinging on acceleration you need to do something more.
2) There are several true "tuners" out there like the SEPST (SE Street tuner is not really a tuner), Mastertune TTS, Dynojet Powervision, and maybe a few more and there are several of what I have been calling piggy back units that some call "fuel controllers" such as the Xieds, Patriot Top Fueler, Vance & Hines Fuel Pack etc.
As stated earlier my preference is for an actual Tuner as opposed to a fuel controller because it will provide a better state of tune than a controller regardless of whether one goes the route of an actual "dyno" tune.
Which "tuner" to buy/use is best determined by......first what your professional dyno tuner recommends which is usually what he is most familiar with....and second which one you think will be easist to use if you don't go the full dyno tune route. There is a wealth of knowledge over at HTT and this is the link to the Tuning section of the forum http://harleytechtalk.org/htt/index.php?board=13.0
There are many dyno tuners (some independent and some working in actual HD shops) that actively post over there and the "debates" between which tuner is best are often pretty heated.
For me personally, I choose the SE product because it was reported that it was a little more user friendly for novices and I'm a total idiot when it comes to this stuff, and my local HD dealer uses the SE stuff. I think most independent shops prefer the TTS or Dynojet products but I don't have an independent tuner in my area so if I ever felt the need for a professional dyno tune I would go to my local HD dealer. So SE for me as it's easier for a dumbass and used by my local and only HD dealer.
3) Cost of stage 1 is really all over the place depending on how far you stray from stock exhaust and "tuner" vs "controller". You can buy a set of slip-ons for anywhere from $200 to $600+ and use the OEM head pipes or you can buy a complete setup from one of severel manufacturers that include head pipes and mufflers. The Switchback comes delivered with a 2-1 setup and personally I would keep it that way. If I owned your bike my preference would be to eliminate the catalytic converter (I think it has one) and go with a complete setup like a Supertrapp Supermeg 2-1 if it's avaialble for your bike. However, from a HD standpont they are relatively quiet and some like a louder system. Been there done that and I perfer just a little louder than stock. A complete system will set you back about $600-800.
Air filter kits are generally between $100 and 200...remember you are buying more than just an air filter as the kit will come with a different mounting setup for increased air flow. You could drop a K&N in the stock air box but by itself it won't do much as the box is restricting flow.
Controller vs. Tuner is probably something like $100-250 vs $350-500.
Yea it all adds up quickly with an EFI bike. Gone are the days of buying cheap slip-ons, an air filter and a few jets but in return, if done smartly, you will have a bike that runs much better than a carb model.
One last point. Most HD guys like to change things after a little while.....different mufflers/headers, add a small bolt-in cam (which by the way can get you around 100 hp and 110tq on that 103" without headwork), go all out with headwork, increased compression and big lift cams. Buying a tuner now like the Powevision, TTS or SEPST, even though not technically needed, will allow you make those changes down the road where a fuel controller that you may have paid $200 for will become completely useless. Just another reason "I" went with a tuner. I spent over $4k to take an 88" 2002 FXST from stock to 109hp/107tq. I can do the same with my 103" RG with just bolt-in cams for less than $1.5K if I have the dealer do all the work and source the parts...something I am considering doing this winter.
Hope that helps and good luck convincing your bride to be.