In order to know which keywords to target, it's essential to not only understand the demand for a given term or phrase, but also the work required to achieve high rankings. If big brands take the top 10 results and you're just starting out on the web, the uphill battle for rankings can take years of effort. This is why it's essential to understand keyword difficulty.
This makes Google Suggest a relevant source for keyword research, it contains a large amount of organic keywords very closely related to a full or partial keyword and can be used to find additional most searched appending keywords that make the whole keyword less competitive. Google Suggest can be researched through the Google Search website or through a compatible browser for a small amount of keywords but also in large scale using free scraper tools.
Most keyowrd databases consist of a small sample of the overall search universe. This means keyword databases tend to skew more toward commercial terms and the core/head industry terms, with slighlty less coverage of the midtail terms. Many rarely searched for longtail terms are not covered due to database size limitations & lack of commercial data around those terms. Plus if those terms were covered, there would be large sampling errors. Google generates over 2 trillion searches per year and claims 15% of their searches are unique. This means they generate searches for over 300 billion unique keywords each year. The good news about limited tail coverage is it means most any keyword we return data on is a keyword with some commercial value to it. And with Google's Rankbrain algorithm, if you rank well on core industry terms then your pages will often tend to rank well for other related tail keywords.
How do I know this? Because I’ve done it! In late 2013 I was still struggling to earn a respectable income online. I started up a keyword research service and marketed it to just this blog audience. It turned out to be quite popular. Basically – I would find niches using Long Tail Pro then create Keyword Research Packages. Each package contained usually several hundred keywords with KC scores calculated. I was selling these packages for anywhere between $97 to $147. I even sold one single package for $597! That one had very high search volumes and very low competition.
Mentioned in the above video is the Keyword Competitiveness (KC) feature within Long Tail Pro. With earlier versions of Long Tail Pro, you could only access this feature if you paid extra to upgrade to the Platinum version of Long Tail Pro. Pricing structures and the way the software is delivered has evolved over the years – and currently (as of the date I am writing this) all features of the software are included no matter which payment plan you choose. So right now – everybody who buys Long Tail Pro has access to this extremely powerful feature.
Given you have a good idea of where to start and are fairly confident you are speaking the same language as your client, jump start research by generating related keyphrases and long tail variants with the ever so easy to use Google Autocomplete. This tool makes predictions based on what you are typing that are a reflection of Google search activity.
The limit on manual keywords could be higher.  I personally wish we could input 10,000 keywords at a time, instead of 200.  However, I understand the costs that Long Tail Pro has to maintain each time a new manual keyword is input.  Not a deal breaker, I just wish the limit was higher. (To be fair, I don't think most keyword tools have a bulk manual option at all).