I started playing this game a month ago, so I just went through the process that you are slogging through right now.
A quick aside before I get into the meat of my response: It became clear to me very quickly that, to be competitive here, you MUST upgrade equipment. That's not to say that you must upgrade anything to have FUN here - you don't - but to be COMPETITIVE, you really do. Yes, I'm sure there are a couple of players who are outliers and serve as exceptions to this, but whatever. The game is effectively rigged to put you at a DISTINCT disadvantage if you choose not to upgrade equipment; to be clear, this isn't a criticism of WGT - kudos to them for figuring out how to monetize their game better than any other game developers I've ever seen.
That said...
I AM competitive; so for me upgrading was a given. I'm also thrifty, so I wanted maximum ROI on credits spent, and I only wanted to upgrade each item in my bag one time for the foreseeable future, so I did A LOT of digging through these forums to help me accomplish my goal. I distilled that information into an action plan that would enable me to upgrade my whole bag for under 6,400 credits. Here's the plan I followed (not necessarily right for everyone, but this is how I did it):
Step 1: Buy the R11 3 wood at L30. It's cheap, it's forgiving, and it's as long as the drivers you can obtain at this level (longer, if you consider that you can also hit it off the deck on par fives and long par fours). You can EASILY use it as your driver until you reach Tour Pro, and you can use it as 3W long after that. It's 395 credits, but only 356 after the 10% discount (I always made sure I already had the credits I needed when I got to the levels where I'd need them, so that I could always take the discount).
Step 2: Upgrade the wedges. I did this at L42, procuring the L42 Gorge 60*, the L41 Gorge 54*, and the L39 ATV 64*. The base distances for those three are 50/75/95 which are the increments I wanted, but YMMV. Cost on the Gorge are 295 each and the ATV is 495. Total for all three, with the discount, comes out to 978 credits. NOTE: I wanted precision wedge play, so I put all three in my bag immediately; this basically means one "extra" wedge in the bag since the starter PW (which you can't remove) has a base yardage of 90 (which makes it worthless with the 95 yard Gorge in the bag), and it means a gap in the longer clubs since you have to lose the hybrid to add the extra wedge. I was OK with this, and in fact it's not hard to learn how to use the R11 3 wood to cover most of the shots in the 190-210 range with a little practice. Again, this won't be right for everyone, but it's how I did it.
Step 3: Add the R1 driver at L48. This club is a true difference maker. Love it. And after hitting the R11 3 wood so much prior to this, I found the R1 really easy to hit (those two clubs are very similar in their "playability"). It's a bit expensive at 2,495 (2,246 with the discount) but you'll be able to use it for a VERY long time.
Step 4: Get the Daytona putter at L55. I was originally going to save putter for last, but I noticed that I was lipping out A LOT of putts with the starter putter, and I couldn't escape the thought that many of those would be falling with a better putter. When I read through the thread on putter mapping and saw just how much more consistent the Daytona was than the starter putter (on true "ding" putts), I knew it was a no-brainer. I've only had it a few days and it has already made a HUGE difference in holing mid-range putts (say 10-25 feet). It only costs 725 credits (653 with the discount) which strikes me as a stupid good value for what you're getting with this in your bag.
Step 5: Close out the upgrades with the G25 irons at L59. I've literally had these in my bag less than 24 hours, so I'm still getting a feeling for them, but the difference between these and the rest of the irons I've hit is stark; the high ball flight makes a big difference in your ability to follow Harvey Penick's advice and "take dead aim". There will be a learning curve, for sure - but after just a couple of rounds I know these are going to cut my "avg. distance to pin" stat considerably. Price is 2,395 credits, which works out to 2,156 with the discount.
Total for everything, after the discount, works out to 6,389 credits and I shouldn't have to upgrade anything other than balls for the foreseeable future. NOTE: I'm still hitting starter balls, but am researching ball upgrades now and will probably start investing in balls soon. I just didn't see the point until I had all of the starter clubs out of my bag.
Whew... :-)