I plan on having 50 units by the end of the year..what's the best/most simple software to use for bookkeeping?

Although I've never used their property management services, I have used "BSU" - Business Services Unlimited, for all my bookkeeping for 2 of my 3 businesses over the years.  

I also have a bookkeeper and accounting firm for all of that. 

Sounds like you might want to think about farming out your bookkeeping both for last year and the future.  

Payroll, bookkeeping, etc can mostly be done remotely.Here is a great list of questions to ask a potential accountant:http://www.biggerpockets.com/forums/51/topics/70447-questions-to-ask-a-cpaAlso check out the www.NAEA.org page in your search.

Odesk for bookkeeping may be a solution I need, entering receipts is so tedious.

If you have a bookkeeper and is OK taking that extra cost, it works, but for me, I want it just like Quickbooks, where I download and tag the transactions and done.3- I found that their online leasing workflow to be top notch.

I forgot to mention I work part-time on the side with a real estate investor who does tax prep and bookkeeping for other real estate investors.

Right now, we're thinking that my husband does the project management and I do the design, bookkeeping and deal analyst.

The longest I remember which is what all my transaction records is 7 years, there are 1 year and 3 years, but I told my bookkeeper to keep everything for 8 years (except those that need to keep permanent like incorporating papers, EIN filings, etc) to be on the safe side.