I've played a lot of RPG over the years but not a terribly large amount of games with Fate mechanics, and those I played didn't have the equivalent of the SU abilities, just handling those with aspects.

I'm fairly comfortable with the basics of social conflict and overcoming obstacles in Fate.

I can pretty quickly grasp the impact of SU abilities on combat, and would feel pretty comfortable that I could field a highly combat effective character (wizard or otherwise), if I spent most of my refresh on abilities that increased stress done, mitigated damage and boosted skill scores.   So naturally my first choice to play in DF was a support/investigative character

My expectation is that a character like this would spend most of his time in combat setting up his more brutally effective allies to succeed, adding aspects to the scene or maneuvers to the enemy that targeted something other than their athletics or might, and then either compelling them at key moments, or passing +2 tags on to ensure that 8 stress attack from the Were-Rhino charge landed.

In most game systems, support characters tend to be ignored compared to flashier threats, but need to be able to handle a bit of collateral damage (like zonewide evocations or burning buildings) and the occasional attempt to focus on them and use threat of their injury/death to skew the battle.  (in D20, it's the "I need to be able to survive one round of full attack, or a bad surprise round").  To some extent just having fate points and some consequence slots free helps quite a bit, but one of the liberating things about a support character is that you can spend resources (skills/refresh) on things other than surviving getting your head torn off.

The trick is to not be so fragile any doof with a bottle can take you out, or that an alpha strike where an enemy mistook you for somebody physically dangerous doesn't kill you outright.   I think the concession rules help, as does "take out != death" sometimes.  But that said, I looked over my character and was just uncomfortable not taking some raw physical combat ability.   (in a Submerged game, he's got fists of 4 and the footwork stunt..he can beat up an average thug, or fight several or go one round with a ghoul without being torn apart instantly, but he'd do much better to find another plan than punching the enemy most days).

Another character in our game was advised to get a little athletics for similar reasons  - she's mostly a social character but you don't  want an average blow from a weapons 2 punk to take you out.

For people who've played support characters....how much is enough?   What are some of the options, especially if your concept doesn't really include athletic or combat training or aptitude?   Is this even a significant consideration in DF?  (it might not be if a small investment doesn't help much in the physical arena beyond the basic mechanics)

Some I've come up with on my own....
1.  Raw athletics pretty high.  Nice if you can fit it in and justify.  Inh speed's bonus counts to the total, as would athletics of a much more reasonable level (say 1-2) and a defensive stunt that raises it a bit in common for you circumstances (like being unarmed/unarmored)
2.  stunt that subs what you're good at as a block vs physical (there's a conviction stunt in Your Story like that)
3.  Some combination of toughness skills to boost a low athletics/endurance score.
4.  for spellcasters, some way of getting passive defenses when you don't have an action to block (see Harry's trenchcoat+boosted athletics as he got deeper into the war...he started out as a wheezy wizard with no defenses if he didn't raise a block)
5.  Veil/Stealth and some kind of way to contribute in a combat that doesn't rely on becoming visible.

For me, the magic number in a "submerged" game was endurance Average (the third stress box matters) and a 4 skill vs most physical attacks.   For the other character, she's got Average end & athletic, inh speed and inh recovery.  She'll get hit more often but recover faster, seems about a wash for me.  She started with Mediocre athletic and was advised to raise it a bit.

What's your experience?  How much defense do you bother with on characters who aren't expected to be so flashy/obviously dangerous in combat that they aren't likely to be focus-fired for long?