I think an aluminum underpanel would be something that's nice to fabricate. The OEM plastic pieces don't really hold well and end up getting destroyed. There are already proven products out there (C-ONE and Beatrush), but it wouldn't hurt to create your own.
For tube bending, I'm not entirely sure what you could do. Headers? CAI or SRI? If the Celica is meant as an experimental car, maybe try to fabricate the parts you would originally want to upgrade--that would probably be the best use of your time and give you relevant modifications.