The book recommends talking to Marel for aging, who supplies that stopping aging is hard, especially in complex organisms! Some simple organisms are essentially immortal, but you need a process of constant renewal and a robust repair system to manage without bringing in serious magic -
Marel appears to have been nerd-sniped. Margaret is going to learn a good bit about the science of aging! That boils down to 'it's possible but expensive and needs to be built into a shape, which should be designed around 'will not die of old age''.
Skin options are easier. Scales, feathers, fur, spines, simple. Chitin you might have to rework your joints. Plant material, slightly complicated to interface with an animal body (and bark and wood require special consideration to have working joints), but also possible. Crystal and stone, difficult, and like wood require special consideration. Metal, fabric, and plastic vary depending on how organic you want your main body and how much you care about having your skin function as a sensory organ. An amorphous blob-person, difficult (the main problem seems to be neurology; the book notes that if you want a slime pet that's cheaper). Fancy options like 'lava contained within forcefields' and 'never-melting ice' are expensive but possible.