Some advice from a nearly 14 year old player here...
Basic relayer advice:
First things first, learn the caps for all stats:
https://www.uoex.net/wiki/Stat_Caps
The only stats that MUST be on a relayer are: SDI, RPD, HP, STR and EP. SDI and RPD are the most important stats after big str/hp.
For most early relayers you want to max all the player stats as well (dex, stam, int and mana). You also want to beef up your regens too. Don't stop adding regen to your relayers until you hit the diminishing returns (values can be found here:
http://www.uoex.net/forum/viewtopic.php?f=1&t=4688). For early relayers get your FC and FCR up as well. Get your elemental resists up as well (NOT Physical resist - just wear a SoT for now, more on this later). Max your HCI.
On the EP argument: I never added it to any relayers... then spent millions in gold and ED replacing all my old relayers that didn't have EP. The reason is that EP is uncapped, any uncapped stat must be abused. Drinking a greater str pot and jumping your HP by a few hundred points is big game vs many areas (Sgail, tanking spec queen, some Xmas quest areas to name a few). Don't forget that str is a factor in your damage calculation, so jumping your str by 100 or so adds huge damage to every hit. EP is uncapped, abuse that fact, don't be a fool like me and have to spend a few years replacing every relayer because you thought it was too annoying to drink a pot from time to time.
A few things to note. SSI reduces the leeches (all 3) on your weapons and once you have 220+ dex you do not need it anyway so why hinder yourself? Don't add it.
Advanced relayer advice:
This last advice is only for those sitting in the 500+ HP club, but it is worth thinking about early when planning your relayers...
There are some nuances for later relayers. Because you are talking about a SoP this advice doesn't apply to this weapon. Once you are all BC/TH this advice comes in, but I will add it for completeness to this post. Endgame players have their physical resist at a MINIMUM (it's the reason one should never relayer a SoT). There are also strong arguments made for not having any DCI as well (I am too old and tired to replace all my relayers with DCI on them just to remove it, maybe one day I will). The reason is when you are late game you WANT to be hit, as often as possible and as hard as possible. RPD is insane. Like, serious insane. Nobody can do 1k+ damage per hit with a weapon. But your RPD can. Your RPD ends up doing similar damage to your weapons in some situations. In some situations it completely replaces them. I wrestle the benedicts (5 at a time in the halloween boss chamber) to death. I don't even bother equipping my 6x60 bow or 7x60 Dreadsword (brag) because the damage I do with my perfect statted endgame weapons is insignificant compared to the damage my armour does with RPD. Next halloween I'll record some short vids showing me doing, sometimes, 5k+ dmg per hit with RPD. There are loads of other places it helps, watching minos one hit themselves in labyrinth etc is funny. Therefore the following stats should never be on any endgame gear: DCI and physical resist on relayers and HLA on weapons (for the same reason). I would accept some arguments for having some DCI for the heavy hitting non-physical mobs (spec queen etc), but if I had a button that would auto remove all DCI from my gear right now, I would press it.