It's fine but the issue with such large enclosures is monitoring them and their feedings. If you're getting a small species then you'll also have issues with finding them, never mind feeding them. You'll need to tong feed or take it out to a smaller enclosure for feedings, or feed in the enclosure with close supervision and ensure they're finding it every time. With so much space you're also going to want A TON of decor clutter, especially the top half and ceiling. They are arboreal and spend 90% of their lives on the top half. If done right and supervised, 5gal is fine, but I'd personally recommend going a tad smaller.

Humidifier isn't really needed. Humidity isn't that big of a deal except molting periods or you have an exotic species. If your enclosure lacks humidity during molting periods it can easily be solved with extra misting, moss or damp paper towels. If you're getting an exotic species then bioactive enclosure is a must. Bioactive enclosures will just naturally have high enough humidity and they won't need extra. Measure humidity and temp inside the enclosure before deciding to get extra heating or humidity. Most of the time you don't need extra heating nor humidity.

No flat top openings. They're arboreal who'll make hammocks on the lid and top openings with tear them apart every time. It's only fine if it's a deep lid such as bigfatphids and amac boxes or you create a hatch in the middle that you can open without touching the lid. You'll also preferably want tall enclosure instead of a wide one. Arachnid shop and tarantula cribs are great examples of suitable enclosure.

We have a guide pinned in this sub, definitely have a read! This guide isn't bad to read into either.

Something for $20 is probably the cheapest you'll find for a ready made enclosure. The ones on amazon are usually the zila micro enclosures which are notorious for escapees, especially with smaller species and younger jumpers. They're fine but you might need to cover the ventilation with mesh and/or cover gaps that are usually at the door. The other good recommendations here are arachnid shop, tarantula cribs and bigfatphids but all of them are on the pricer side, bigfatphids being the cheapest. Amac boxes are great to DYI into an enclosure and any other food container turned upside (so opening is at the bottom) with holes drilled/poked in will do great too:)