Personal loan to consolidate credit/add credit line?

I have a very limited credit history. I am a co-signer on my car loan which is about to be paid off. I wanted to buy a house but my score is between 600 and 620 and too low for a USDA 100% finance loan. Mortgage lenders suggested that I get secured cards so I've had two for about 2-3 years now. One just raised my limit from $250 to $750 because of my good history. I keep my balances low on the cards but this only raised my score from the high 500's to where it is now.

To my collections: I only have two. Both from surgeries when I was very broke. One is $500 and the other is $2400. Lender say the $2400 medical collection is killing my credit.

My car is going to be paid off in the next 3 months then I lose that line of credit and my score will likely drop. I have been considering a personal loan of $5,000. This will add a line of credit to replace my car loan, allow me to pay the medical collections and have some money for my upcoming wedding.

Does this seem like a good way to keep my credit history going and consolidate and remove the bad debt? My credit advisers say that 3 lines of credit aren't much for my history, so as I am about to lose one, I need to replace it. I don't want more credit cards either. :(

Read more: Personal loan to consolidate credit/add credit line?