New rental rehab... plumbing issues

[Todd Brown]

Closed on this properties last Friday. They got me good... I had been told that plumbing issues had been fixed. Looked like the drains had been replaced. While working on the property I flushed an upstairs toilet and noticed the downstairs restroom drains gurgle. The downstairs doesn't drain well ... after looking into things found only the upstairs shower is properly vented. I didn't budget to replace plumbing on this house. Has anyone experienced venting issues like this and is It advised to use oneway/auto vent fittings or do I need to have things completly repiped. Any help is appreciated. I had a couple posts about this property. I'm brand new and trying to figure things out as I go. Check those posts out if your interested... I'm just hoping that I'm not in over my head.