Will I be able to purchase items from steam market after additional credit transfer?

There is no delay . when i add steam credits i am able to purchase instantly so there will should be no issues.

The only difference is if it is the first time you have used the card to add funds. in which case it is only a few days delay as displayed here.

My full answer is there will never be a 30 day delay from adding funds. and the only delay is on the first time not everytime

Edit: the other option is to buy a prepaid steam card and then redeem the credit, this will take instant affect and you won't receive a delay. these can be bought it approved shops and from some online sites. can't tell you exactly where without knowing where you live but a quick google search will let you know.