With mortgages, there are two basic areas of specialism, being Residential and Buy-to-let Mortgages. These are the two basic categories of mortgages and you at least must make sure the intermediary you select is experienced in the particular area you want. If the mortgage is for your own home then you need a residential mortgage specialist, and if it is for buy-to-let then you need a specialist in buy-to-let mortgages. Of course, some can deal in both and that is fine as long as they are specialists in each area as opposed to just being able to provide you with a mortgage.
Additionally, make sure they specialise in your particular area of specific mortgage requirement as closely as possible. In other words, they have done deals for similar properties for similar rental purposes in the recent past. Sometimes this is not possible if you are looking for a very special niche-lending requirement. However, in most cases they should have dealt with the broader category of what you are looking to lend the money for. You can only find this out by discussing the details with them and gauging their responses.
In addition to the mortgage broker or advisor being a specialist in buy-to-let mortgage lending, it is also preferable that they are familiar with buy-to-let investing strategies; this is so they can help advise you on what you want to achieve. When you are first starting out investing, maybe this is not that important as you will not be looking at advanced approaches to lending and using the more creative strategies. However, you should be looking to detect how well they are able to understand what you want and offer lending solutions to the more difficult projects so that you can be comfortable they will add significant value in your business relationship.
However, absolutely make sure you use a whole of market intermediary who has access to every deal available and specifically ask for this. Being whole of market means they are truly able to be independent in giving you advice for your situation on which is the best product currently available in the whole of the market.
If you need a broker recommendation, one who ticks all the boxes for the best brokers as stated in the book, I can let you know who I am currently using as my main broker if you drop me a request for this information at email@example.com