External mass mailings question

I have only been a part of this list for about a week and I havent seen 
this question brought up, if this questions as been answered many times 
before, I apologize.
My company has about 20 servers that will send what I call "day to day 
work related" messages. I think I know how to setup my SPF record for 
these. But I found out that we use many (varies month to month) external 
servers. These are used for Marketing purposes. We use some very well 
established companies (salesforce.com(gotmail.com)) and some not so well 
established (as far as I can tell). I see that these mass mailings 
(MM's) have a "from" and "return" address to my domain, but, obviously, 
they are not sent from my domain. I feel if I allow (in my SPF record) 
these domains to send on behave of my domain, since I don t know what 
else they might be sending out, is a bad idea. I don t want to associate 
my domain in an SPF record with them. Also, If I just allow their IP's 
then my TXT file will be huge (I already have 20 IPs listed). The larger 
companies we use have farms of 10-20 servers that "may" send out our 
MM's.  Just for the record, we only send messages to people who OPT IN 
and are very good about removing anyone from our lists, truely we are! ;-)
If I were to add all the potential IPs that could send messages for my 
company (internal & external) it could reach 60+ IP's. Does this sound 
normal?
What/how, in your opinion would be the best approach for a situation 
like this? I am really new to SPF, so if my question above is too 
generic or makes no sense to you SPF gurus, just let me know.
I did see a few panels at the Inbox Conf discussing SPF and at the time 
I was very skeptic of this, and to be honest I still am. But I wanted to 
at least get involved and publish my records. Its better than nothing.
TIA,
Nurv