I had a blast conversiong bladeguards from stormcast miniatures and I just had to jump into making a sternguard unit. If you want to see the bladeguard conversion, please go here:
https://www.dakkadakka.com/dakkaforum/posts/list/793314.page#10989307
For the sternguard I picked up 10 easy to build stormcast minis in Ebay, plus some shoulderpads and helmets from actual sternguard and got to build them.
My hole army is going to have primaris-sized infantry, but keeping first born elements. Not only it looks cooler and can make for some conversion work to make the minis more personal, it is also a way to use the same mini for multiple purposes.
Each of this pairs are made out of the same easy to build miniature. Was quite fun to come up with ways to make them look different. There was a particular harder one that had a female body, much thinner than the other, which needed a change on the upper body, and biggening of the legs and hips.
Last guy lacks the head of the thunderhammer since I still waiting for the bits to come through the mail :(
Some attention to the back of the minis too, having them packed with all the weapons they need to please the emperor. Backpacks, combi-weapons and bases are not glued for easier painting.
Since sternguard, sargents, etc get to use combiweapons, I got 6 of the minis equiped with a combi-weapon set up that can be swapped. This are converted intercessor bolter rifles, and I still need to add the magnets to in.
I have done this in a similar way for my tactical marines with special weapons, but with another method. That stops me from swaping special weapons from one unit to another, thus more work since I have to do 2 sets of special weapons, but it does have a better look on the completed mini.
BUT...
Something that might be noticeable when compared to my non-elite infantry is the size of the minis:
Sternguard left, Tactical marine-intercessor right. The sternguard are a bit bigger, but the most obvious difference is base size.
This could have some tabletop implications, but I believe it to be worth it. Here there is a comparison of squashed untis, and although there is a difference, dont think would be a problem outisde of tournament competitive matches. On the positive side, though, not obly makes the mini look so much better, but it also fits if I want to use them as characters, such as captains, luitenants, or even sargents.
Side note
Looking back I am quite happy with how I am progressing on the hobbie. Almost 10 years ago I builded the right mini as a captain (not painted), and It gets smashed by the new version
Same goes for painting. Left my first ultramarine ever painted, right a bellow average intercessor I painted a couple years ago: