I got a lite weight 522 sd for Christmas.
So far its been a blast and very reliable.
I'd like to do some mods to it, and the first thing i would like is to sbr it.
Is the gun a good candidate for an sbr? I'm worried about longevity of the plastic receiver.
I don't want to spend $200 on the stamp and have a big paperweight if it breaks.
How easy is it to get replacement parts or have it repaired if something happens?
Or am i worrying over nothing?

Any advice is appreciated.

ATI does not have a part# for a 9" barrel for the lite weight, even though they do sell a LW pistol.
i'm still waiting on a reply from them. progress documented in this thread.
you could cut/re-crown the barrel. that would be around $150-$200, as far as i know.

are you concerned about the reliability of the plastic receiver in general, or in SBR form?
i'm thinking if it's good for 16" barrel, it should be as good, if not better, for 9".

I'm concerned with the plastic receiver in general.

I have zero experience with these rifles. I was just wondering if there known problems with the plastic receivers.
I have a plastic ar, and it has been good to go. I'm just hoping the same for the plastic on these.

i do not have any information about the reliability of the receiver.
i can tell you that when it's entirely dis-assembled and the two halves are separated i did feel that the plastic is rather flimsy. it felt like a $15 toy rather than a $300 registered firearm.
but in assembled form with all the metal parts inside and out, it seems to hold up well.

parts and accessories can be attained from ATI's online store.
for items not listed you can call them with a part# attained from one of the exploded view diagrams.
aftermarket and upgrades do exist. i think one of the more popular sites is hkparts.

i suggest calling ATI's customer service and discussing your concerns.