Best power shaper accessories according to redditors

Nah.
Here are tools that will get you going well, IMO.

http://www.amazon.com/Xuron-410T-Tapered-Tip-Shear/dp/B000IBQEEW/ref=pd_sim_sbs_469_4?ie=UTF8&dpID=31aHJgL3pNL&dpSrc=sims&preST=_AC_UL160_SR160%2C160_&refRID=1QA091E0TTJ7J8FW3MKN

http://www.amazon.com/X-ACTO-Series-Knife-Cap-XZ3601/dp/B005KRSWM6/ref=pd_bxgy_469_img_2?ie=UTF8&refRID=0QC3ZT6985MJV0ZTB72N

http://www.amazon.com/Piece-Precision-Needle-File-Set/dp/B006N3EPKE/ref=sr_1_3?ie=UTF8&qid=1453474293&sr=8-3&keywords=needle+files

are you using pins? pinning your fabrics really helps. when pinning, make sure all the edges line up nicely and the fabric lies flat. cut off any excess. to know where you need to stop sewing and pivot, mark with a pencil (or similar, but don't use markers that will be visible on the outside. if you notice the fabric layers shifting a lot while sewing, it might be an idea to invest in a walking foot.

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also, clip off all excess fabric at the corsers after sewing and if you don't own one already, a corner shaper will really help getting the corners turned nicely.

Yes. It's the specific knife that was used on the shaper. Something like this:
http://www.amazon.com/Jesada-CIK-040-Knife-Combination-Cutterhead/dp/B00004SXX1

This is what we've got:

• Been tempted to try the GW clippers, but these have worked fantastically. https://www.amazon.com/Xuron-410T-Tapered-Tip-Shear/dp/B000IBQEEW/
  
• Rarely ever use knives as we use the GW mold line remover. It's safer, as in won't cut you. https://www.amazon.com/dp/B00MYUK8VK/
  
• Really like the GW plastic glue for model asembly. It works and sets fast. Sometimes you need a superglue and PVA (elmers style white glue) for basing.
  

Sorry for being late, but maybe check out /r/gunpla. I've seen that model on there a couple times. I guess you could start out with a cheap sprue cutter and the sanding sticks that have been mentioned. Maybe grab a HG gunpla model to practice on first, since I think they are put together sort of similar.