You may run into the fact that most lenders will only allow you to borrow up to 75% of the appraised value on an investment property so do you numbers and make sure it makes sense.

Yes, you will have to make up the difference if it doesn't appraise, unless you're able to find a bank that will refinance at a higher LTV (90%) which might be extremely difficult to find. 

For example, you may only be able to refinance out 75% of your appraised value or 100% of what you paid as listed on your HUD, whichever is LOWER. 

By law, banks cannot make a mortgage of over 80% of the appraisal.

Here's the situation:We currently own a house and our loan is $320,000 but the house is appraising at about $400,000+.

If after all this you sill don't feel comfortable you could have an appraiser take a look. 

The home appraised for $410k and we closed in February 2021.

Negotiations, using the right forms to keep you out of trouble, chasing people to get those forms signed on time, disclosures, comps, wrangling inspectors and re-negotiations and appraisers, managing deadlines - and the list goes on and on.The fact that a good Realtor makes it look easy doesn't mean it is. 

When buying a property like this, whether for rental income or other use you need to have it appraised.

I offered above asking price, property appraised at exactly the offer price.