Selenium is toxic at as little as 5mg a day, RDA is 3mg/day from memory, so most feeds won't supplement more than 1mg a day at recommended feeding rates as this plus naturally occurring is normally sufficient. Its a difficult one too as the symptoms for deficiency and over dose are the same, if you felt you did need to supplement more because of high workload (as it helps with muscle recovery) you can test your grass/forage or have bloods pulled to evaluate and supplement safely.
Vit E RDA's are something like 1000IU for a well adult horse, 2000-3000IU for young or old and up to 5000IU for a horse in recovery from something, and those with things like PSSM or other muscle issues have been fed up to 10,000IU with no ill effect and positive improvements. Vit E cannot be made in the body and is normally acquired from fresh grass, it significantly depletes once forage is preserved.
I feed a balancer that provides 1mg selenium and 100oIU of vit E, however with PPID there is the thing about oxaditive stress contributing to it and the horses having lower immunity and being in a state of constant conversance and vit e (and selenium) are good antioxidants. So about a month ago I increased Jess' vit E supplementation and she now gets about 3500IU (Jess gets very little access to fresh grass so her need in supplements is higher than the same horse who does get grass), I specifically got one which did not include selenium as I know she gets 1mg selenium from her balancer already and likely more from her forage and science says that if there is a deficiency of one the other (between vit e and selenium) can compensate so I would rather increase vit E as it doesn't have the possibility of toxic effect. I am still tweeking her diet as her feet show little signs of non specific (not sure what) dietary deficiency, I still get a little flaring, some mild event lines and thrush at the drop of a hat. Her recent foot growth is coming in tighter and with less rippling, its going to be another 4-5 months before I can see if it reduces flare. She has however also been on haylage for the last 2 months, so that could also be effecting it, but I am now doing 50% hay/haylage so I would likely see a change back if it were that.
When buying vit E, go for natural vit e not synthetic, they both work, but synthetic is less readily absorbed so you need to feed 2-3 times the amount for the same uptake. Natural E is more expensive but as you feed a lot less it works out better. I brought mine from Progressive earth in the end, I don't think they were specifically the cheapest but I also needed some magnesium so lumping together for postage worked out for me. You can also get it from forage plus and a few other places, even a health food shop
ETA, I shared a website a while back that gave general values for different minerals for the UK, you could have a look there and see if your area is high or low for selenium to give you a rough idea.
ETA, when I was at a nutrition talk last year I asked the independent nutritionist how to tell a good balancer/vit&min sup and her answer was based ont he Vit E content, if it had a good dose (over 1000mg) of natural E it probably was a good one because Vit E is the most expensive thing for them to add