What testnet environment and configuration for Bitcoin? Every tutorial I have gone through to download bitcoind, I haven't been able to complete because my computer can't handle the full node download.

These are my specs:
2017
Processor - 2.3 GHz Intel Core i5
Memory - 8 GB
Storage: 50.8 GB available.

What's the lightest way I can run testnet so I can develop applications / hardware with the protocol but not buy new hardware (can't afford it)?

Many thanks for any suggestions.
You can run your node in pruned mode if storage is your concern. Reduce Storage

If you want to try to use bitcoin on testnet you can use Electrum wallet.
Be sure to download it from their official website https://electrum.org/. (Do not download Electrum from another source than electrum.org, and learn to verify GPG signatures.)