yes, it will, it will determine how much backlinks that point to your site, the more you get, the ranking your site also higher, but all depends what niche you're in.
Your question is really very interesting. I have already discussed about the topic with many blogger friends. If your blog is dofollow then you should take precaution of your outbound link, if your outbound links point to some spammy websites then it might be harm of your blog, next I have see that most of people just copy some content from other and paste it on the comment form, so that means your blog will effect for duplicate content, which google hate more. Again if your blog is dofollow then your outbound link will be more due to the comment link, so it might be less value for search engine. For search engine ranking you should always take care for all of these for your dofollow blog, then no problem for get rank on search engine.
Don't know if i am allowed to say which software i used to SEO my websites but if an admin or moderator tells me i can..i will of course mention it's name. The license costs between 250$-450$ depending on your needs. Believe me it's really unbelievable and you learn a lot of things from it.
Nofollow flag will not affect your ranking or indexing in general. So it's up to you if you want to keep in follow or nofollow but I strongly recommend that you have moderation on, so you approve every single comment. As blog4fun mentioned you have to be careful with your outbound links but it's not really important if they are follow or nofollow, if you link to bad neighbourhoods.
If you take care of your outbound link and the writing of comment then I think its not bad for you to make your blog dofollow. Also there are some advantage of dofollow blogs like 1. You get more traffic 2. You get more subscriber 3. You get more discussion on your blog by which you can also learn what your reader want and this might be one good idea to write the next article.