If you want Cories sand is the only way to go - for just the reasons you've mentioned
I wouldn't use CO2 unless you also intend using uprated lighting - with standard lighting it shouldn't be needed.
If your looking at similar stock to last time then without CO2 you could set up using some of the less demanding plants. All would appreciate Vallis, Java Ferns and Java Moss and the Gourami in particular would like some floating plants for cover. I'm using Amazon Frogbit for cover in my shrimp tank and it does well with no special additions to the water. Many of the crypts do fine without CO2, too. Just put root tabs under the root feeders and maybe use a weekly general plant fertiliser for the floating plants - Seachem Flourish or similar.
Play sand is fine and very cheap. Needs a very good wash, and will need regular stirring to prevent it compacting and producing toxic gases. Malaysian Trumpet Snails are a good addition in tanks with sand as they burrow through it and keep it aerated. If you want any let me know as I always have some spare - free snails, just pay p&p
You can have half and half sand and gravel , but will need to use something to actually separate the two areas otherwise it won't take long for them to end up mixed together.
You'll need a good soft sand and something inert that won't raise your pH as you want to stock Cories and other soft water fish. Be careful as often white sand will be high in carbonates and push the pH. Also white sand shows every speck of dirt and grows algae rather well
This should do the job. Not cheap compared to play sand, but is a good make and claims not to affect pH.