[Vespers War]

Even if South Korea decides to stick to "defensive" weaponry, they license-built over a thousand Vulcan Air Defense Systems. A couple hundred were vehicle-mounted as the K263, based on the K200 IFV, and around a thousand were left as towed anti-aircraft guns. The K263 uses the same turret as the M163 that the Americans built based on the M113.

Any that South Korea isn't planning on using for their own SHORAD needs would likely be very welcome in Ukraine as supplements to existing systems, and they've been largely replaced by 35mm Skyguard systems, both towed and mounted on the K30 Biho. The Houthi in Yemen have fitted VADS into the beds of pickup trucks, and I have to imagine Ukraine would be capable of doing the same.

On the missile side of air defense, Korea is replacing a combination of Stinger, Igla, Mistral, and (UK) Javelin systems with the indigenous K-SAM, so any or all of those could be considered for transfer. I don't think they have enough of their larger SAM systems to consider transferring any of those.

If they're willing to provide offensive weapons, the K9 Thunder is probably too new, but it's replacing around a thousand of the K55/K55A1 self-propelled artillery system, which is based on the M109A2 Paladin. There are also around 100 of the K136 130/131mm MLRS in reserves that were originally going to be sold to the Philippines, but they backed out of the deal last year. Moving away from vehicles, there are a couple hundred ATACMS missiles that Korea might or might not be willing to send, along with a couple hundred obsolete Hyunmoo-1 short-ranged ballistic missiles that have allegedly been kept in reserve (which have around the same range as ATACMS, so they wouldn't be an escalation with regards to how far into Russia Ukraine could strike).