I maxed out Overwatch in my last playthrough. That's my main driving point of my long post:This was very good summary of certain approach and something I must return later, but for now:
Actually no. I'm playing on Normal difficulty and have Tech 7. So far things have worked though there has been some annoyances, like difficulty to find Epic / Purple pants with a mods slot, this is due certain clothing mod that is only available as Epic and very convenient during certain thing with Delamain.
Other annoyance is Overwatch that is too expensive to keep upgrading which is an issue because it's only suppressed sniper rifle in game. Otherwise, for Body focused builds there are good stuff around.
For clothing, there are head wear available in world and shops with mod slots also Epic and Legendary ones with two slots for shirts and jackets at least.
Cost of anything is irrelevant if you properly spec into Tech; eddies are infinite by the nature of being able to craft and sell things that cost a lot less per unit to make (by buying the components directly from the same vendor). That and a lot of what you craft and upgrade is free.
As I also mentioned, in my current play through (tech 14 so no crafting legendaries) I am only missing one legendary slot (the head; you suggest you can buy legendary head items a slot but honestly go check again, they're extremely scarse and I have yet to find one and that's also ignoring the main thing that irritates me about headwear in this game: most of it looks awful and the craftable legendary head looks good) and I'm only level 19. So yes, it's not essential (as I said )
If absolutely maxing out a build isn't on your agenda, tech can be ignored. If it is, you must invest in it. But it has other benefits too (such as eddies and being able to rapidly obtain good gear without needing to hunt it down). And in normal difficulty, I honestly don't think any build matters, by the way. You can randomly select stuff and the game is emminently beatable so I'd say that in this case it comes down to what you enjoy the most.