You can just buy an old cheap laptop with the said characteristics...
I think OP is looking for a more cost-effective solution. Something equally good as a hardware wallet but probably cheaper. he didn't say cheaper, but I am pretty sure that is what he meant. He doesn't own a hardware wallet yet and buying any kind of laptop is a more expensive investment than a hardware wallet.
If OP stored Bitcoin only he could consider generating a paper wallet, but since he has other coins as well, a hardware wallet is the best balance between price, security, and ease of use.
I don't think so. There are $50 cheap old laptop in the local market here in the Philippines which could serve its purpose. And if the OP wishes to buy a hardware wallet overseas rather than here in the Philippines then he would pay a good amount of tax and considering the new memorandum of the Optical Media Board here, the shipment would be delayed a little bit.
How about Mycelium? It is easy to install and security is decent.
More secure than custodial wallet? Yes, but the security hugely depends on security practice/awareness of the owner.
That's what I thought.