My question is what are good endless spells for a seraphon army and which endless spells are really bad
Burning head isn't bad. Purple sun is just useful for thinning stubborn units like Mortek Guard and Hearthguard.
Burning head is nice, if you have 40 pts spare and is going monster-heavy. Emerald lifeswarm could be nice in TQ lists, if it wasn't so expected. Otherwise, geminids, balewind and purple sun are top-3.
i am messing around with modelling options for being able to use different endless spells. I cant see on GW, but is the purple sun on the large monster oval? spell portals are 50mm? 32mm?
Most endless spells have their own, sometimes unique, basesize thats why in the base size errata just says "use model"