First order of advice..NEVER trust the employees at Petco. Just because they work in a pet store, doesn't mean they know anything about pets. At least that has been my experience.
You can keep a betta in a 1 gallon, however many people will go ballistic if you mention it. It's more work then a bigger tank because you will need to do more water changes to keep the ammonia levels down. Most people will tell you they need at least 2.5 gallons.
I think it's easier to get one of those sets that comes with a lid, light and filter but they tend to be $$$, at least here they are.
When I do that if I get one with a lid, lights and a heater that has plenty of plants and things to hide in would that be ok for my betta?
Yup, that would make a happy betta :)
They are tropical so they need a heater and some hiding places like silk plants and a castle or a cave is good. I have one betta that has spongebob's pinapple house and I have never seen him go in it and another that loves his little castle tower. And a theromoter to check the water temp in case the heater spazes out. FYI - Tetra 50 watt heaters don't work for crap. It barely keeps a 3 gallon above 73
Also I would get some water conditioner, to make tap water healthy for the fishie. I have used betta bowl plus and stress coat before with good results. There are plenty others but those are the ones I used.
And are all bettas at most petcos going to be unhealthy?
It depends on the store, but lately my petco has gone downhaill in betta care. My experience with both walmart and petco is that unless the fish has just arrived, they are not going to be in perfect health. However, that doesn't mean the fish is going to die in a week. Look for a fish that swims around if you poke the cup and isn't gasping for air or having rapid gill movements. If it's been in the store awhile, chances are it has a higher then normal ammonia reading and some fresh water will do it a world of good. They may have holes in their fins caused by either ammonia burns or fin rot. The fins will grow back with proper treatment (warm water, frequent water changes and aquarium salt).
If you are using a filter, it has to cycle. I dont use filters, but I think there is a sticky with how to cycle a tank.
Oh and be sure to acclimate the betta to the water temp in the tank everytime you do a water change or it may go into shock and die.
LIDS - I highly recommend lids. They can jump out of the tank, bowl, ect of there is no lid. I had one do that a few weeks ago :(