Obviously email / chat to DJ’s to check they understand what music you want. The agreed fee should include everything; PA and lighting. DJ may add on some travel expenses if an overnight stay is involved or its over 50 miles away, this should be agreed at time of the booking.
The more you can help the DJ the better as you know who is attending whereas he will be meeting them for the first time. Obviously good DJs will suss the crowd out anyway but any pre advice on what you KNOW would go down great with your friends helps. Think about who is attending and what you think everyone will like – maybe there are some classic tunes you and your friends used to jump around to at uni? The guys too, usually have some shared “guilty pleasures” from back in the day!
Just because you have a wide age range of attendees doesn’t mean you can’t have some belting tunes later on, a 4/5 hour gig should allow for you to have great tunes as well as some funky classics earlier on for the whole crowd. So it doesn’t have to be “cheese” central all night as there’s plenty of good stuff from the 50s to now to play! DJs need PLI and Patt tested equipment and may need to email this to the venue.
As for the venue; check what time music ends and be wary of any vagueness in that area!! Do they have a set time when the music has to be turned down because of other residents staying in the hotel? Is there a problem with nearby residents or farmers (“…the music is upsetting the cows!…”) Is there a DB meter installed and does it automatically cut off the electricity?
If they have a db meter and you like your dance and indie played at a bar/club volume then consider another venue as it’s often set too low or not taking into account all the guests soaking up the sound. Djs generally arrive around 6- 6.45 to set up for the evening party and may require couple of covered tables from the venue. If you feel the room lacks atmosphere and there are chunks missing out of the portable dance floor (oh yes) you may want to look into hiring in some “up lights”. The venue may have a local supplier for this.