Layer 3 switch over the router

Generally, most of the switches can't be used as WAN routers since NAT is not supported on switches. Juniper EX4300 does not support NAT. If you have  virtual private LAN L2/L3 service provided by ISP link between locations and NAT is performed on some other device than you can use EX as CE device (or if you are using public IP addresses on all your end devices).
Problem can also be any other protocol that is needed and not supported on EX device.
Layer 3 Protocols Not Supported on EX Series Switches

There is absolutely no problem configuring the ports as layer-3. Delete family ethernet switching and configure them for family inet, on the other hand, if you wanted to only have ethernet switching, configure port as access and configure an vlan.x as an l3 interface, follow the below link and revert for any help.

https://www.juniper.net/documentation/en_US/junos/topics/reference/general/ex-series-l3-protocols-supported.html

-R

I'd buy a Cisco 1841 off ebay for about £40.  Use that until the equipment comes.  A simple NAT config is dead easy.

1841 model?
Does it have Gb fibre ports for SFP LC mmode?

Sorry, no, I didn't take note of the gbic requirement

There is no need for SFP on temporary router. You can use SFP on switch just to bring connection with ISP up.

[ISP router] --- fiber optic --- [Juniper SFP ge-0/1/3] -- vlanX-- [router WAN interface gi0/0 - Juniper ge-0/0/23] --- [router inside interface gi0/1- Juniper ge-0/0/22]

Explanation (Juniper 24 ports switch; Cisco 2 port router):

configure Juniper switch ports ge-0/1/3 and ge-0/0/23 to belong to the same VLAN
on SFP port ge-0/1/3 connect link with ISP
on RJ45 port ge-0/0/23 connect WAN router's interface (e.g fa0/0)
This will create point-to-point link between PE and CE routers (it will work except in the case that subnet mask is /31)

Router's inside interface (fa0/1) connect to port ge-0/0/22
Configuration of ports depends on your switch configuration and future plans. But, generally, can be configured as point to point link/VLAN (default route point to router as next hop) or as trunk (subinterfaces for each VLAN on router's side - trunk on Juniper). Point-to point is preferred.

I thought the switch cannot do NAT  and no IPSEC.  Need firewall for that.  I don't have it either yet.

Thank you!  I see you indicated a PE router.  In my case the ISP doesn't provide any routers.  I indicated I will provide my own router.