First off work out the guest list - no point in looking at venues until you know how many people they are inviting. Too many times I know of couples who fell in love with a venue only to realise it couldn’t accommodate the number of people they wanted when they finally put their list together.
Re venues - is it being used for the ceremony as well? If so check it has a licence or arrange for a legal ceremony elsewhere. Also does it have somewhere that the bride can get ready/have make up checked/quiet space after the ceremony with your son - might not be necessary but it’s nice to have.
Check on when you have access - depending on the level of decor getting in on the day may not give enough time. And when is chucking out time - if they want to go on later you usually have to pay for an extension.
Note that venue wedding coordinators are not wedding planners - they are there for the interests of the venue not to help you plan or deal with external suppliers - ie if the DJ is late that’s not their problem to solve.
Catering - the venue may have in-house catering but be aware that they can be very inflexible as to what you have can (ie 3 course sit down only, etc). Some venues may be “dry hire” where you can bring in your own caterers or ones they have approved which can be great and gives more flexibility but may also cost more as they might need to bring in kitchen equipment/tables/chairs etc. Insist that you have a tasting whichever one you go for, including wines.
Speaking of booze - most venues/caterers will allow you to bring your own in but will likely charge corkage. Discuss this with them and see which works best.
Sound & AV - is the venue set up for speeches/DJ/dancing etc? Many have this on site but you might need to bring it in - the venue will be able to help you with this usually.
Some venues may insist that you use their preferred suppliers for catering/AV/florals - just be aware of this if the couple want to use anyone specific.
Send Save the Dates sooner rather than later! And start the table plan as early as possible - I recommend starting with everyone invited and then removing people - I’ve had couples doing theirs at 3am the night before before their wedding not realising how tricky it can be. My Perfect Table Plan is a great site for managing this and the guest list as a whole.
Hitched is a great website for tips as well as for finding suppliers. The Knot is great for wedding websites.
If the budget allows I would recommend getting an “on the day” planner (depending on how complicated things are obviously). Usually you hand everything over to them a couple of weeks before and they then manage everything - ticking off those last few bits and ensuring it all runs smoothly on the day so that everyone can enjoy the wedding.
I hope this helps a little and my best advice is to take it step by step - get the big ticket items in place and then the fun bits start.