Haswell to Broadwell? That's 4th to 5th gen and the socket is still the same which means you don't have to swap the mobo if you go for this path, but then it is not worth to do this upgrade because the improvement is not insignificant at all, same goes to Kaby Lake (7th gen) upgrade, AMD Ryzen is a better pick if you want something that can bring major performance improvement to your rig.
Your budget is not enough to get a decent quality PSU, RM400 is the minimum recommended budget for that purpose

Total: RM 2415 (E TECH price inclusive of GST)
AMD Ryzen 5 1400 - RM 819
ASRock AB350M Pro4 - RM 399 (This is a mATX mobo, if you prefer full ATX mobo to be installed in ATX casing, please swap to ATX model instead, top up is required)
G.SKill Aegis 16GB (8GB x 2) DDR4 2400MHz RAM - RM 570
Superflower Leadex Gold 550W - RM 449
Segotep K7 - RM 359

Tempered glass casing doesn't affect airflow, it's more towards aesthetic purpose, more and more big brands are releasing their own tempered glass casing nowadays and if you would like to jump on the bandwagon but have limited amount of budget then K7 is among the cheapest option out there. If you aren't fancy of tempered glass then there are plenty of other casings to choose from within your budget like NZXT S340/Corsair 270R/CM Masterbox 5 etc. 

As for the PSU, no doubt it's gonna be Leadex Gold, it is a tier 1 PSU, the Corsair CX series is barely considered as a good PSU (tier 5 with expensive price tag  ), another option that you can consider is EVGA G2 (obsoleted)/G3 (new).

Strix B250 is more towards aesthetic as it comes with some fancy features like RGB support to sync up the RGB fans in your casing, you can save a couple of bucks by going for a cheaper B250 if you aren't going to utilize those extra features by the Strix B250 model and use the money for other parts.

Ryzen is good so far, it's a complete new process node and architecture. i5 7500's clock speed is slightly higher than 7400, if clock speed is important for you (single threaded apps which rely on single core performance will perform better on a CPU with higher clock speed), then yep I would top up for the 7500.
AM4 ATX mobo starts at around RM 550, another model that you can consider is Asus B350 Prime, red & black board design as well.

Ryzen 5 1400 is like an i7, so 30% performance improvement should be easy to achieve, but if you don't mind adding another RM 200, Ryzen 5 1600 is a good pick (6 cores 12 threads), more powerful and a great leap from your current i5 

If you want to keep the level/grade of Leadex Gold which is a tier 1 PSU, then you will have to look for EVGA G3, CM V series (not VSM), Corsair HX, RMx, RMi etc, G3's price is closest to Leadex Gold's price.

All the PSU listed by you are okay for overclocking, the internal components that build up the whole PSU matter the most, it is recommended to fork out more budget on a decent quality PSU for stability purpose.

FYI,

SilverStone Strider Essential - tier 6
Corsair TXM - tier 2