I need to start by stating nothing exists which matches 100% of your criteria, in all instances. That being said, Id be glad to offer up some alternatives that come as close as possible...
First idea that comes to mind is to liquidize your concentrate, and vape it out of an e-cig style device, such as an Ego (etc). I'm sure @
BLAZING OG can give you specific details as to what brand/parts you need, but It'll all end up being $25 or less I'd think. Once you have that built out, you can do your own custom mix of PG/VG and whatever concentrate you want, or you can just buy this stuff and mix it in:
http://www.liquidizer.com/ejmix.html
I've not used it, but I hear it works very well. In any case, you will need to get "replaceable atomizers or whatever" once in a while. Bonus, though... lots of people (around here anyway) use e-cig style devices to vape nicotine e-juice all the time. Youll blend right in with them and you can adjust things to disguise smell and taste.
The second thought is for you to use something like the Ophos or Omicron-Lite:
http://www.w9tech.com/omicron-lite-essential-oil-vaporizer/
http://www.w9tech.com/o-phos-essential-oil-vaporizer/
They are both very stealth, and the traditional carts are all self contained so once its loaded you never see or smell what went in there until you vape it again (or stick it 1" from your nose). Again, though, you can only reuse those carts a few times before you need to use a new one (although 200-300 good size puffs are to be had out of each fill). Plus they can leak if you dont use/store them properly. Then again nothing is "leak proof". Ever.
Last, but not least you can go with a LYAG (load as you go) pen/heater/cart like the VB Dabbler/DR. Dabber style pens. You could load the bowl/heater in private, take it outside, vape whats in it... and come back with it empty. Issue here is that you will only get 2-3 good hits before you need to reload... and yes you still need to replace the heaters once in a while. That also reminds me, W9 Tech also makes these style carts for their Ophos and Omicron-Lite line of vape pens. Just another option... AND you can rebuild their heaters.
Anyway, all these suggestions are under $100, so you need to weigh out whats most important to you and make your choice.
Also, not all of these devices work "best" with all types of concentrate. What do you usually use? BHOs? Shatters or more oil? So on and such stuff...