As far as overdosing, if you do the EI (Estimative Index) method of dosing, you will not overdose. If you do a search for "EI" you will get tons of info on it. The basic concept is you dose the ferts listed in the chart per your tank size and every week you do a 50% water change and this will level or start fresh your tank for another week of dosing without the worry of overdosing or doing a bunch of testing. I do this and it works out great!
If you want to save money, go with the dry ferts. The flourish products are good but will burn a hole in your pocket compared to the amount of dry you can get and how far it will go. The only flourish product I am using is Flourish but I only dose 5ml of this 3X a week so it last me awhile. If I can be of any other help, let me know.