I would suggest if you're serious about going the tech route, skip the PADI weekend AOW course, and take the full individual specialty courses instead. Nitrox, Deep, Night / Limited Visibility, Rescue. All of these are fine with single tank backmount, and the Deep class may be an opportunity to add a pony bottle to your rig.
I suggest starting with a backplate and wing system, since that will be the foundation for most training after recreational. A BP&W has interchangeable parts that will allow switching between single and double tanks. If you ever train with GUE, that will require a BP&W system, as they don't use sidemount.
A sidemount rig is fun and very comfortable in open water, but really comes into its own in tight overhead spaces (caves). It's not popular for charter boat diving unless most of the boat is tech divers with similar setups. I've been known to dive single tank sidemount off a liveaboard, and that works Ok with a boat full of recreational divers.
I would suggest adding sidemount when you are transitioning into technical training. TDI Advanced Wreck, Advanced Nitrox or Cavern would be good courses to do in sidemount. Prior to that it would be good to take a separate Sidemount course and be well versed in neutral buoyancy and technical, anti-silting fin kicks.
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