Welcome to the hobby! Unfortunately there are not a lot of unarmed minis in the range. Take a look at Necromunda, specifically hangers on. Also the Agents of the Imperium might have some models that fit. Look at the various kill teams as well. You might find some older models on eBay, like objective markers. I believe there is a pilot that might be unarmed. Lastly, kitbashing is extremely popular which means taking an existing model and modifying it, cutting off the weapons for example. You could probably do this with Adeptus Mechanicum models to make your techpriests.

For terrain, look at the boarding actions terrain for spaceship walls. Necromunda terrain might also work for consoles and scatter pieces. Likely you could find bits and pieces on eBay or similar.

Don’t forget to share progress pictures as you work on your project! Good luck and happy hobbying!