You'll have to excuse Harry, he has a thing about editing
. Which is not to say he is wrong, releasing a poorly edited book
may cast a lasting stigma even over your future work. That said, many authors do publish their first books without professional help, sometimes coming back later to fix their first efforts if/when they get any income from their work to justify the expense. BUT, if you are unable to get professional help, at least look for good outside help.
Don't assume you can do it all yourself.
Much can the same can be said about the cover.
To your questions...
There is no requirement to go with just one (unless you sign up for special offers requiring exclusivity for a limited time (make sure it is limited) - eg: Amazon KDP Select).
Again I agree with Harry, Amazon has to be on the top of your list. Make up your own mind whether their exclusive option would suit you (you only have to be exclusive for a few months, so it may do).
Other options to seriously consider include Smashwords and BookBaby, as these will distribute to many resellers for you (read the conditions etc. on the respective sites). Both those will deliver to Amazon too, theoretically saving you some effort, but I suggest you go direct to Amazon, it can save hassles later (words of experience here). There are others, but these are two I have used. I was with BookBaby for a while, but moved to Smashwords when BookBaby changed their offerings - but even so, I think BookBaby is worth considering. BookBaby is flashier, and easier to deal with if you aren't in America (but slow to respond to emails). While I have found Smashwords to be more generally helpful; their website isn't as flashy but is more functional.
Further advice is hard to give without knowing more. If the genre is non-fiction or specific types of fiction then your alternatives can be different to most popular fiction. If you have the knowledge to create your own epub (for example), then you may pick a slightly different approach than if you just want to upload a Word file.
DRM is a potentially contentious issue that regularly raises long discussions here (usually in other parts of the forum, thankfully). I chose
not to use it, and my first reaction would be to advise you do the same, but it is a personal choice. Read the arguments and make your own decision. Again, your choice can also be influenced by the genre. For example, specialist non-fiction may see more benefit from DRM.