Welcome along!
Something generic like that sounds fine. You may wish to accessorise it a touch with a belt, or a hat, or whatever, but that sounds like a simple and generic costume with which to spectate for a bit 
As long as you avoid: obviously branded stuff, camoflage gear, jeans and trainers, you’ll be fine.
A plain shirt, with a tunic over the top (to hide buttons etc), plain trousers, sturdy boots/shoes, should be fine. Poncho and scarf is likely a good idea, given the time of year.
If your intent is to wander around and get a feel for the game, I can also reccomend belt (simple leather job for instance) to hang things off and hold the tunic in, and maybe a tankard (plain pewter), as you may get offered a drink or two.
Should you want to pick up any extras, there are several shops and traders on site, and I can reccomend Chows Emporium for accessories.
If you’re just wanting to wander round and chat to people, some sort of backstory if anyone asks is likely helpful. Oo, here’s one:
“I’m a messenger from Varushka, I was dropping off parcels here and thought I’d stay and see Anvil for a while before heading home. What’s been going on?”
People love to talk about their characters and what they’ve been doing and what is important to them. You will likely see many sights, but if you’re going for an exploratory ramble, may I reccomend seeing:
The Imperial Regio (magic stone circle)
The Senate (politics building)
The Glory Square of Dawn (duelling ground, many flags)
The Navarri camp (especially if you can make the storytelling in the evening)
And the bars/pubs/tea-shops. Of which there are many
Happy to answer any further questions!