It is hard to say in my opinion. I have seen different versions of 2A3 and they all have different Voltage ..... sigh. I got a hard time finding a "suitable" transformer.
I "think" the best way is to find a good 2A3 circuit first as there are so many "modified" ones around. A good one with high resemblance to the "original" may help to save you trouble in the future in case you need to replace the transformer (assuming you are talking about the Power Transformer).
If you use Point to Point soldering (搭棚), you can change anything anytime provided you construct the set well enough. If you use PCB, the PCB will have to be good enough. Else, you can cause severe damage to the PCB and troubles will then come.
I am relatively new too, just been here DIYing for less than a year. But I learned a lot from the friends around here.
There are people around here who has knowledge of good circuits. If the circuit design is good, it is worth upgrading. Else it will be a waste of money. Putting in good components MAY just "magnify" the weakness of the equipment, instead of curing it.
I strongly agree to what Cheaplow said. As he is more experience, he is able to give you more insight.
Since you said you are new, I would suggest you listening to different tube amplifiers and figure out what combinations you want before upgrading anything. I like 2A3 myself but I have to admit that there are other amp and pre-amp that does not fit my taste. It is like choosing a car. Better test drive (listen with different combinations of speaker, etc) before you actually upgrade it.
When you attend the class, tell Modi Sir and alike what type of music you like best. KK Sir has good knowledge of those electronics stuffs and Modi Sir is very experienced in Tube Stuffs. Willaim Sir can arrange for different package deal for you. Therefore, you have a great team serving you there in the class. ENJOY!
My suggestion is the same as Cheaplow. Attend the class first and tell them what music you like and what are your current settings. They can then suggest you to get / try whatever you want.
You don't really need friends with lots of Hi Fi stuffs. You can go to the shops, right? The price there may not suit your budget but at least you know how they sound like. A DIY project usually cost less than 1/2 of the street price of any good brand product. This is my impression.
I checked with my colleague who got a 2A3 as well. He also said that it is not easy to find matching speakers as the power (output) is not that high.
The last time I listen to a 2A3, it was driving a medium size speaker (can't recall the brand). The sound quality is good but it just lacks the "punch" in my opinion.
DIY is always fun. Just go step by step and don't be afraid of making mistakes. Ten years from now, when you look back, I am sure you will be proud of yourself. I myself feel very happy with the products I finished so far ... :
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