+1 252+ Atk Wellspring Mask Tera Water Ogerpon-Wellspring-Tera Power Whip vs. 120 HP / 248 Def Volcanion: 255-301 (77 - 90.9%) -- guaranteed 2HKO
I tera water'd just to see if it made a difference and it didnt. 120 HP/248 Def saves 20 EVs over 248 HP/140 Def while maintaining the same damage values, and I forgot to make it a Bold nature like a dumbass.
+1 252+ Atk Wellspring Mask Tera Water Ogerpon-Wellspring-Tera Power Whip vs. 120 HP / 248+ Def Volcanion: 232-274 (70 - 82.7%) -- guaranteed 2HKO
40 SpA Choice Specs Volcanion Earth Power vs. 252 HP / 212+ SpD Heatran: 360-424 (93.2 - 109.8%) -- 56.3% chance to OHKO
If a 50% chance to OHKO is acceptable, then 120HP/248Def/40SpA/100Spe is the breakdown for you. If it isn't, which I get, then I'm kinda out of options, since a guaranteed OHKO against Heatran takes 148 SpA EVs and Specs, which your budget cannot handle at all.
As for Wogerpon, while it wouldn't wanna come in on 40 SpA Choice Specs Volcanion Sludge Wave vs. 0 HP / 0 SpD Ogerpon-Wellspring: 274-324 (91 - 107.6%) -- 43.8% chance to OHKO, it isn't the best option since it outspeeds you anyways next turn, or that turn even if you got the free switch somehow.
Wish I could help more.