If I put down 50% and he puts down 50%, where and how do the funds join together and get transferred for the purchase.Once the house is purchased, what costs must be taken into account?

The prior owner's certificates do not transfer to you.

Or are they transferring larger sums from other retirement accounts to get going?

Are there any legal/financial ramifications if I collect rent through my personal Venmo account, then transfer to my business account for my rental property?

He's offering to transfer ownership of it for $500, but I can get a more generic domain for $20.

For instance, Closing fee will be around $1,200Lender's Title will be around $1,000 (or less)Recording fees will be around $140 - Cant say if you will have to pay transfer tax to the City because you have not indicated which suburb the property is in. 

Can I simply transfer my IRAs into my Solo 401K?

Copies of third party reports related to the ownership transfer, if necessary 12.

Some people will buy a property subject to the existing financing which will transfer title into there name and leave the loan in the previous title holders name, but the new owner will continue to make payments for the old owner who's name is still on the financing/loan.

You wire transfer that amount, the the  bank will then execute and record a Release in the county records.