currently on Unifi 100Mbps plan, plan to upgrade current ,modem/ router to replace old D-link DIR850L. Dont plan to get WIfi mesh, prefer one single device to power up the whole house .... Can anyone suggest any good router / modem below RM150 to cover 2 storey terrace ?

I think it would be quite hard, but can search for tp link or dlink i suppose. Doubt so can find a router which can support a 2 storey house with that budget, maybe around 200-250 can bro. IMO la.. Not so sure, but can search for those 2 brands. Currently using Asus AC-RT86U as main router, now also selling at RM400 online, can cover even up to outside my house gate for 5Ghz connection. 2.4Ghz i think further. Can reach up to my room, main router downstairs and still have 4 bar wifi strength.

with limited budget i can just suggest renew contract and get wifi 6 from tm

alternatively look for mercusys product. it's budget sub brand of tp link

can start here

https://forum.lowyat.net/topic/4658096

May I know how to know the 'penetration power' of the wifi router ? Which specific reading I should take note of ?

Honestly Im no router expert, i just assumed a medium range priced router would do it.. But my house has 2 routers. xD One for my room with my PCs. And one for general, parents and guests when they come. haha. So no one messes with my router lel xD but dont get me wrong, my AX56U can give me range outside my house up to 3/4 houses away.. lol. 5Ghz speed.

Impressive, only 2 small short antenna on the modem

Yah. I was surprised too. Hahah. AX56U second router and Ac86U main router. Haha.

Came across both Dlink X1870 and Archer A6 by TP-Link ... Forget about the wifi 6 compatibility, which router offer better signal coverage ? When it comes to signal coverage, what's the specific reading I shall take note of ?

for coverage you need to check whether it comes with dedicated FEM (PA and LNA). you can look up for these if you not too familiar with that

if wifi 6 is not important at all, can check out Mercusys AC1900 MR50G (by tp-link) because 3x3 antenna is better than usual 2x2 budget model yet within your limited budget (on flash sales of course)

Saw Tenda AC23 about same price with Mercusys , will you recommend it ober Mercuysys MR50G ? PRice about the same .. Number of antenna doesnt determine the wifi coverage right? Which value I shall take note of when it comes to wifi signal coverage ?

nope. tenda AC23 platform is no good enough https://www.acwifi.net/8338.html