The first step is to define your mission.
What are you wanting or hoping you can do and achieve with drone use?
This definition is where it all starts.
Your Objective will, in many ways, dictate what payloads the drone needs to carry.
As an industry we are being spoilt at the moment by the huge array of sensors available, their dimishing size and weight, and most importantly, their dimishing costs.
The distance you want to cover is a prime factor in the choice of systems.
The distinctions that exist in the hobby-grade drones between multi-rotors and fixed wing start to blur when you get to larger systems.
For example; Adding a hybrid power system to your multi rotor can increase its flight time to as much as 4 hours. cost: $7KAUD + 4.2kg. The off the shelf copter for this weight is $20KAUD.
Consider then much more affordable drones so can have multiple drones for multiple jobs.
Fixed wing drones still cover the longest distances.
Multi-rotors can hover and decend closer to targets on demand.
The terrain can play a role in how you use drones and in some cases, which drones are more suitable.
A major decission factor is 'required loiter time' over targets and the required height over the target to get the detail of data required.
The payload you choose can have a big bearing on these requirements.
'Custom Designed' sounds a lot fancier than it is.
Because payloads are so varied, as varied as the jobs being done, no one design of copter can cope with them all.
The core of the design revolves around the weight of the payload, firstly, and then any particular poyload requirement, such as, 360deg rotation so legs have to be retractable.
With fixed wing planes you traditionally had to have spare wings and tail surfaces on hand to replace those that inevitable break on landings.
Now with VTOL fixed wing this problem is solved.
But all builds have to revolve around the payload.
The airframes are surprisingly simple for multirotors and as such can be easily built with a minimum of replacement components.
The goal is to make the builds as modular and easy to service as possible.
Lightness, strength, payload security, vibration tolerance, flight times are all inherent in the design/build cycle.
Only high quality components can be tollerated in any area of the builds.
UAVS has control of these aspects to greatly reduce your risk of breakdown or failure.
You get a vehicle Fit for Purpose.
Reduce Cost of Ownership.
Knowledgable and friendly advice.
Local assistance from UAVS.