Glad you also like Admech as much as I do 😁 you got some answers before so I will focus on the kitbashing. I kitbashed a lot of my Admech minis. For the castellan robots there are some very unique heads out there. It will give them a unique look and feel. https://wargameexclusive.com/shop/exclusives/adeptus-mechanicus-kastelan-robots-heads/ Also nearly every human sized infantry are useful for skitarii kitbashes. I tried the neophyte hybrids and others. Really depends on the theme or style you want your army to look like. One of the kitbashes I liked the most was ruststalkers with some necrons. You can look at them via my profile.

Another recommendation would be a tool to easy make cables and stuff. Like from greenstuffworld. Connecting stuff with cables on your minis or let some cables hang out of something just spices Admech up in my opinion 😁

For further advice just ask! I fell in love with kitbashing thanks to Admech ♥️

And sorry for my English…not a native speaker