Hello, 

1.-for that kind of workload anything with 4 cores and 8gb of RAM will do the trick, the thing here is to configure properly your Postgres database engine, you can easily do it here: https://pgtune.leopard.in.ua/#/ in any case select the one that fits to your budget.

another crucial part for your configuration is the workers, please take a look at this comment of one of the top Odoo guys on how to setup the workers of your instance: https://github.com/odoo/odoo/issues/39825#issuecomment-555256475

2.- Apps are irrelevant, the number of users is what matters, take a look at the previous link above.

3.- Ubuntu Server with LTS is always the best way to go.

hope it helps.