How to develop a buying strategy?

Smite was the first MOBA game I played. After trying the various game modes on the Xbox One version I stuck with the straight forward Arena game mode. I enabled the auto-buy features and just dedicated my gameplay to getting stuck into the action.

Over time I started to play more Joust and Siege games. I experimented with disabling the auto-buy but just didn't seem to find a way of deciding which items to purchase. I reverted back to auto-buy and haven't really given that choice a second thought until...

I started playing some Conquest matches. The matches appear to be a lot longer, with levelling being slower. I know feel at a disadvantage because I am kind of locked in to certain attributes for a longer period of time than in other game modes. For example, if the auto-buy upgraded an ability I rarely used it would only be a minute before my favoured ability was upgraded.

What should I do to decide on a suitable build? Is it worth sitting in spawn for 20-30 second intervals deciding on which item to purchase? Should I really reconsider the auto-buy option?

Usually auto-buy and similar helpers (like preconstructed decks for a trading card game) are really great if you're new to the game, because you're able to just start playing without having to worry about strategies. Once you become a more advanced player however, you probably want to use a different strategy or adapt your playstyle to the current situation.
For League of Legends for example there are many sites dedicated to item builds, I only know one for Smite, although it should be easy to find more you should use to get a general idea of how a character should be built.

The most important thing to know about developing your own strategies and playstyles is though that you will fail at the beginning. You will be slower than the auto-buy function.
But once you've practiced making quick decisions and know which abilities you will use you can adapt to situations very fast, which is very important in MOBAs.

If I am playing a new character in my MOBA (LoL) I usually follow these steps:

1. Play the character
   Before I start thinking about a build, I've read all abilities and played the character at least once, so I know the "feel" of its kit. I mostly build things I deem strong on this type of character but since Smite has an auto-buy function, you may use that as well.
2. Develop your own build
   Now, that I'm comfortable with the characters playstyle I try to find a build that works for me in most games. I have to admit though, that I sometimes skip this step and continue with the next. (Which will speed up the process a lot if you're just starting out.)
3. Read other peoples builds
   Other people - especially pros - already have invested hundreds of hours into playing this character (unless its just being released, although there probably are people who played it in its beta phase), so why not use their knowledge of the characters skill set to my advantage? This step is great to find new ideas on how a character could be played, in LoL for example there is the character Morgana, who can be either a very strong supportive champion or a bursting mage, depending on the items you buy and the skill order.
4. Refine your own build
   At this point I'm comfortable with the character, have a general idea what I would build and I know and played what others would build. Now I should be able to find a build that I can play every game right? Wrong - your build has to be adaptable to a difficult situation. If your build has a lot of physical defense and you have to face enemies that can deal a lot of magical damage, you will have to at least change the order you buy your items. Updates and balance patches might also affect your build extremely, from making it extremely overpowered to so underpowered that it cannot be played anymore, which will force you to change some of the items you regularly use.

As you may have noticed, I usually don't play Smite. But the general process I described above applies to most MOBAs and other games that allow you to adapt your character/deck/... like MTG. As long as you follow these steps and some basic ground rules, which are pretty much common sense (e.g. don't build only defensive stats on a character meant to deal a massive amount of damage) you should be able to come up with a build that fits your playstyle and makes you perform a lot better than the predefined builds meant to help you get started.

It depends on why you're playing. If you just want to have some casual fun, auto-buy is fine. If you want to be competitive or have finer control over the game, auto-buy is a terrible option.

You're concerned that it will take 20-30 seconds to figure out what to buy each time you go to the fountain. That is true, but only for a little while. As you keep playing and gain more familiarity with the items, you will spend less and less time figuring out what you need to buy and you will be spending 1-2 seconds in the fountain instead of 20-30; however long it takes you to just select what you want and be on your way.

There is a ramp-up time, but it's worth getting through it so that you have better control over your stats in game. If the options are truly overwhelming at any point, there is still a recommended list of items you can cheat off of.